Transcendental devotional service has five stages of reciprocation:
1. The self-realization stage just after liberation from material bondage is called the śānta, or neutral stage.
2. After that, when there is development of transcendental knowledge of the Lord's internal opulences, the devotee engages himself in the dāsya stage.
3. By further development of the dāsya stage, a respectful fraternity with the Lord develops, and above that a feeling of friendship on equal terms becomes manifest. Both these stages are called sākhya stage, or devotional service in friendship.
(Srimad Bhagavatam------introduction).

There is a specific difference between the pure devotee and the confidential devotee. Different potencies of the Lord are engaged in serving the Supreme Lord in different transcendental relationships. They are situated in conjugal love, in parental affection, in friendship and in servitude. By impartially judging, one can find that the internal potencies of the Supreme Lord who are engaged in conjugal love with the Lord are the best of all devotees. Thus both internal devotees and confidential devotees are attracted by the conjugal love of the Supreme Absolute Truth. These are the most confidential devotees of Lord Caitanya. Other pure devotees, who are more or less attached to Śrī Nityānanda Prabhu and Advaita Prabhu, are attracted by other transcendental relationships, such as parental affection, friendship and servitorship. When such devotees are attached to the activities of Lord Caitanya, they at once become confidential devotees in conjugal love with the Supreme Lord.
(Teachings of Lord Caitanya).

The particular rasas (flavors or tastes) of the devotees who merge into that ocean of bhakti-rasāmṛta are known as neutrality, servitorship, friendship, parenthood and conjugal love. Conjugal love is very prominent, and it is symptomized by the devotee's decorating his body to attract Kṛṣṇa. The flavor of servitorship increases to include affection, anger, fraternity and attachment. The flavor of friendship increases to include affection, anger, fraternity, attachment and devotion, and in parenthood the attachment increases to include affection, anger, fraternity, attachment, and devotion. There are also special flavors experienced in friendship with the Supreme Lord, and these are manifested by friends such as Subala, whose devotion increases up to the point of bhāva.
(Teachings of Lord Caitanya).

The sum total of all these rasas is called affection or love. Primarily, such signs of love are manifested in adoration, service, friendship, paternal affection, and conjugal love. And when these five are absent, love is present indirectly in anger, wonder, comedy, chivalry, fear, shock and so on. For example, when a man is in love with a woman, the rasa is called conjugal love. But when such love affairs are disturbed there may be wonder, anger, shock, or even horror. Sometimes love affairs between two persons culminate in ghastly murder scenes. Such rasas are displayed between man and man and between animal and animal. There is no possibility of an exchange or rasa between a man and an animal or between a man and any other species of living beings within the material world.
(Srimad Bhagavatam------1:1:3------purport).

Śukadeva Gosvāmī was already situated in the liberated stage, yet he was attracted by the pastimes of the Lord. This proves that the quality of His pastimes has nothing to do with material affinity. Similarly, the young cowherd damsels were attracted by the bodily features of the Lord, and Rukmiṇī was attracted by hearing about the glories of the Lord. Lord Kṛṣṇa attracts even the mind of the goddess of fortune. He attracts, in special cases, the minds of all young girls. He attracts the minds of the elderly ladies by paternal affection. He attracts the mind of the male in the humors of servitude and friendship.
(Srimad Bhagavatam------1:7:10------purport).

Devotional service in friendship can be divided into two categories: the first is to act as the confidential servant of the Lord, and the other is to act as the well-wisher of the Lord. The devotee who has confidence in devotional service to the Lord systematically follows the rules and regulations, with the faith that he will achieve the platform of transcendental life. The second type of devotional friendship is to become a well-wisher of the Supreme Personality of Godhead. In Bhagavad-gītā it is said that the Lord accepts a preacher as the most dear servant. Anyone who is preaching the confidential message of the Gītā to the people in general is so dear to Kṛṣṇa that no one can be equal to him in human society.
(Nectar of Devotion).

Śrī Rūpa Gosvāmī says that a neophyte devotee who has simply developed a slight love of Godhead is certainly a prospective candidate for devotional service. When he becomes firmly fixed in such devotional service, that assured status becomes a confidential part of his devotional service.
Sometimes it is found that a pure devotee lies down in the temple of the Lord in order to serve Him as a confidential friend. Such friendly behavior of a devotee may be accepted as rāgānugā, or spontaneous. Although, according to regulative principles, no one can lie down in the temple of the Supreme Personality of Godhead, this spontaneous love of Godhead may be grouped under devotional service in friendship.
(Nectar of Devotion).

By sincere efforts to hear Śrīmad-Bhāgavatam one realizes his constitutional relationship with the Lord in the transcendental humor of servitude, friendship, paternal affection or conjugal love, and by such self-realization one becomes situated at once in the transcendental loving service of the Lord. Not only were all the pure devotees like Nārada self-realized souls, but they were engaged in preaching work automatically by spiritual impetus, and thus they delivered many poor souls entangled in the material modes. They became so powerful because they sincerely followed the Bhāgavatam principles by regular hearing and worshiping.

(Srimad Bhagavatam------2:8:5------purport).

Although there is friendship between Kṛṣṇa and the cowherd boys, this friendship is different from the aiśvarya friendship between Kṛṣṇa and Arjuna. When Arjuna saw the viśva-rūpa, the gigantic universal form of the Lord, he was afraid for having treated Kṛṣṇa as an ordinary friend; therefore he begged Kṛṣṇa's pardon. However, the cowherd boys who are friends of Kṛṣṇa in Vṛndāvana sometimes ride on the shoulders of Kṛṣṇa. They treat Kṛṣṇa equally, just as they treat one another, and they are never afraid of Him, nor do they ever beg His pardon. Thus the rāga-mārga, or bhāgavata-mārga, friendship exists on a higher platform with Kṛṣṇa, namely the platform of vipralambha friendship. Paternal friendship, paternal service and conjugal service are visible in the Vṛndāvana rāga-mārga relationships.
(Srimad Bhagavatam------4:24:45-46------purport).

Imperfect life is realized in material existence, in five different relationships we share with everyone within the material world: neutrality, servitorship, friendship, filial love and amorous love between husband and wife or lover and beloved. These five enjoyable relationships within the material world are perverted reflections of relationships with the Absolute Personality of Godhead in the transcendental nature. That Absolute Personality, Śrī Kṛṣṇa, descends to revive the five eternally existing relationships. Thus He manifests His transcendental pastimes in Vraja so that people may be attracted into that sphere of activities and leave aside their imitation relationships with the mundane. Then, after fully exhibiting all such activities, the Lord disappears.
(Sri Caitanya Caritamrta------1:3:12-------purport).

Special natural appreciation of the descriptions of a particular pastime of Godhead indicates the constitutional position of a living entity. Adoration, servitorship, friendship, parental affection and conjugal love are the five primary relationships with Kṛṣṇa. The highest perfectional stage of the conjugal relationship, enriched by many sentiments, gives the maximum relishable mellow to the devotee.
The Lord appears in different incarnations—as a fish, tortoise and boar, as Paraśurāma, Lord Rāma, Buddha and so on—to reciprocate the different appreciations of living entities in different stages of evolution. The conjugal relationship of amorous love called parakīya-rasa is the unparalleled perfection of love exhibited by Lord Kṛṣṇa and His devotees.
(Sri Caitanya Caritamrta------1:4:34-------purport).

The nine kinds of devotional service are hearing about Kṛṣṇa, chanting about Him, remembering Him, offering service to His lotus feet, offering Him worship in the temple, offering prayers to Him, working as His servant, making friendship with Him and unreservedly surrendering to Him. One can directly approach the Supreme Personality of Godhead simply by executing these nine kinds of devotional service, of which hearing about the Lord is the most important (śravaṇādi). Śrī Caitanya Mahāprabhu has very favorably stressed the importance of this process of hearing. According to His method, if people are simply given a chance to hear about Kṛṣṇa, certainly they will gradually develop their dormant awareness, or love of Godhead.
(Sri Caitanya Caritamrta------1:7:141-------purport).

The Lord is the supreme maintainer of everyone and the chief personality among all personalities. In Vrajabhūmi Kṛṣṇa is certainly the central point of love, but no one knows Him there as the Supreme Personality of Godhead. Rather, a person may know Him as a friend, son, lover or master. In any case, the center is Kṛṣṇa. The inhabitants of Vrajabhūmi are related to the Lord in servitude, friendship, parental love and conjugal love. A person engaged in devotional service may accept any one of these transcendental relationships, which are known as mellows. When such a person reaches the perfectional stage, he returns home, back to Kṛṣṇa, in his pure spiritual identity.
(Sri Caitanya Caritamrta------2:9:131-------purport).

Devotional service can control the Supreme Personality of Godhead, who is the supreme controller of all universal affairs. The five rasas (mellows) in the transcendental world are practiced by the inhabitants of Goloka Vṛndāvana in neutrality, servitorship, friendship, parental affection and conjugal love. All these please the Lord so much that He is controlled by the devotees. For instance, mother Yaśodā was so advanced in devotional service that Kṛṣṇa agreed to be controlled by her stick. In other words, the five principal mellows are so great and glorious that they are able to control the Supreme Personality of Godhead. In the material world, however, the so-called siddhis, or perfections, manifest their brightness only as long as one is not interested in devotional service. In other words, the perfection of the karmīs, jñānīs, yogīs and others remains attractive only as long as one does not come to the point of devotional service, which is so great and significant that it can control the supreme controller, Kṛṣṇa.
(Sri Caitanya Caritamrta------2:19:165-------purport).