Bhakti-yoga is the system that the Gītā recommends above all others. Generally, the bhakti-yogīs are engaged in five different ways: (1) śānta-bhakta, engaged in devotional service in neutrality; (2) dāsya-bhakta, engaged in devotional service as servant; (3) sakhya-bhakta, engaged as friend; (4) vātsalya-bhakta, engaged as parent; and (5) mādhurya-bhakta, engaged as conjugal lover of the Supreme Lord. In any of these ways, the pure devotee is always constantly engaged in the transcendental loving service of the Supreme Lord and cannot forget the Supreme Lord, and so for him the Lord is easily attained. A pure devotee cannot forget the Supreme Lord for a moment, and similarly the Supreme Lord cannot forget His pure devotee for a moment. This is the great blessing of the Kṛṣṇa conscious process of chanting the mahā-mantra—Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.
(Bhagavad-Gita------8:14------purport).

Kṛṣṇa once had to carry out an order of Yudhiṣṭhira's by carrying a letter Yudhiṣṭhira had written to Duryodhana regarding peace negotiations. Similarly, He also became the chariot driver of Arjuna. This illustrates that in bhakti-yoga there is a relationship established between the Supreme Personality of Godhead and the devotee. Such a relationship is established in the transcendental mellows known as dāsya, sakhya, vātsalya and mādhurya. If a devotee wants simple liberation, he gets it very easily from the Supreme Personality of Godhead, as confirmed by Bilvamaṅgala Ṭhākura. Muktiḥ svayaṁ mukulitāñjali sevate ’smān: for a devotee, mukti is not very important because mukti is always standing on his doorstep waiting to serve him in some way. A devotee, therefore, must be attracted by the behavior of the inhabitants of Vṛndāvana, who live in a relationship with Kṛṣṇa. The land, water, cows, trees and flowers serve Kṛṣṇa in śānta-rasa, His servants serve Him in dāsya-rasa, and His cowherd friends serve Him in sakhya-rasa. Similarly, the elder gopīs and gopas serve Kṛṣṇa as father and mother, uncle and other relatives, and the young gopīs, the cowherd girls, serve Kṛṣṇa in conjugal love.
(Sri Caitanya Caritamrta------1:8:19------purport).

As long as loving service is rendered to the Lord in the master-servant relationship, there is some fear, for the servant is always afraid of the master, despite the intimacy of self-interest. In this stage the servant is always afraid of the master and respectful of Him. When the devotee is further advanced, he has nothing to fear. He considers the Lord and himself on an equal level. At such a time, the devotee is fully convinced that Lord Kṛṣṇa is a friend and cannot at all be dissatisfied if the devotee lives with Him on an equal level. This understanding is called viśrambha, that is, devoid of a respectful attitude. When this attitude is chosen, it becomes sakhya-prema, or love of Godhead in friendship. On this stage there is developed consciousness of equality between the Lord and the devotee.
(Sri Caitanya Caritamrta------2:8:74------purport).

Goloka Vṛndāvana is the highest planet in the spiritual world. In order to go to the spiritual world after penetrating the covering of the material universe, one must penetrate Brahma-loka, the spiritual effulgence. Then one can come to the Goloka Vṛndāvana planet. There are also other planets in the spiritual world, called Vaikuṇṭha planets, and on these planets Lord Nārāyaṇa is worshiped with awe and veneration. On these planets śānta-rasa is prevalent, and some of the devotees are also connected with the Supreme Personality of Godhead in dāsya-rasa, the mellow of servitorship. As far as the mellow of fraternity is concerned, in Vaikuṇṭha this rasa is represented by gaurava-sakhya, friendship in awe and veneration. The other fraternity rasa, exhibited as viśrambha (friendship in equality), is found in the Goloka Vṛndāvana planet. Above that is service to the Lord in vātsalya-rasa (parental love), and above all is the relationship with the Lord in the mādhurya-rasa (conjugal love). These five rasas are fully exhibited in the spiritual world in one's relationship with the Lord. Therefore in the spiritual world the bhakti creeper finds its resting place at the lotus feet of Kṛṣṇa.
(Sri Caitanya Caritamrta------2:19:154------purport).

Sakhya-rati is described in the Bhakti-rasāmṛta-sindhu (2.5.30) as follows:
ye syus tulyā mukundasya te sakhāyaḥ satāṁ matāḥ
sāmyād viśrambha-rūpaiṣāṁ ratiḥ sakhyam ihocyate
According to the opinion of advanced devotees and learned scholars, a devotee in sakhya-rati feels equal to the Supreme Personality of Godhead. This is a relationship in friendship. Due to having a friendly relationship with the Lord, not only is one free from material attachment, but one believes in equal dealings with the Supreme Personality of Godhead. This is called sakhya-rati. The sakhya-rati devotee is so advanced that he treats the Lord on an equal level and even exchanges joking words with Him. Although one is never equal to the Supreme Personality of Godhead, the sakhya-rati devotee feels equal to the Lord, and he does not feel guilty because of this. Usually it is offensive to consider oneself equal to the Lord. The Māyāvādīs, for example, consider themselves equal to the Lord, but such feelings entail bereavement because they are material. Sakhya-rati, however, is a feeling experienced in the mind by a pure devotee, and he is eternally related with the Supreme Personality of Godhead in that feeling.
(Sri Caitanya Caritamrta------2:19:183-184------purport).

Sakhya-bhakti-rasa is described as follows in the Bhakti-rasāmṛta sindhu (3.3.1):
sthāyi-bhāvo vibhāvādyaiḥ sakhyam ātmocitair iha
nītaś citte satāṁ puṣṭiṁ rasaḥ preyān udīryate
"According to one's original consciousness, ecstatic emotions may be exhibited as continuously existing in fraternity. When this stage of Kṛṣṇa consciousness is mature, it is called preyo-rasa or sakhya-bhakti-rasa."
(Sri Caitanya Caritamrta------2:19:185------purport).

In the third stage, called sakhya-rati, transcendental love is developed, and one associates with the Supreme on an equal level of love and respect. As this stage is further developed, there is joking and such relaxed exchanges as laughing and so on. On this level there are fraternal exchanges with the Supreme Person, and one is free from all bondage. At this stage one practically forgets his inferior position as a living entity, but at the same time he has the greatest respect for the Supreme Person.
(Teachings of Lord Caitanya).

Transcendental qualities are certainly present in the śānta-rasa and dāsya-rasa, but beyond these there is another quality, confidential attachment, which is pure transcendental love. This loving confidence in the Supreme Personality is technically known as viśrambha. On the platform of viśrambha, fraternity, there is no sense of awe or veneration towards the Supreme Personality of Godhead. Thus in the transcendental fraternal relationship known as sakhya-rasa, there are three transcendental characteristics: the sense of greatness, the sense of service, and the sense of intimacy without awe or veneration. Thus in the sakhya-rasa, the relationship of fraternity, the transcendental qualities are further increased.
(Teachings of Lord Caitanya).

The Lord instructed Śrīla Rūpa Gosvāmī to write the transcendental literature named Bhakti-rasāmṛta-sindhu, the science of devotional service, and indicate therein the substance of these five transcendental relationships. It is explained in that great literature how the transcendental relationship of śānta-rasa, taking the shape of unflinching faith in Kṛṣṇa, is further developed into dāsya-rasa with the spirit of service, and then to sakhya-rasa or undeterred fraternity, and further to the transcendental platform of paternal love, wherein one feels himself to be maintaining the Lord. All these relationships culminate on the highest platform of conjugal love (madhura-rasa), wherein all these transcendental relationships exist simultaneously.
(Teachings of Lord Caitanya).

"Rāgānuga-bhakti, or spontaneous devotional service, can be executed in the śānta-rasa when one aspires to be like Kṛṣṇa's cows or the stick or flute in the hand of Kṛṣṇa, or the flowers around Kṛṣṇa's neck. In the dāsya-rasa one follows in the footsteps of servants like Citraka, Patraka or Raktaka. In the friendly sakhya-rasa one can become a friend like Baladeva, Śrīdāmā or Sudāmā. In the vātsalya-rasa, characterized by parental affection, one can become like Nanda Mahārāja and Yaśodā, and in the mādhurya-rasa, characterized by conjugal love, one can become like Śrīmatī Rādhārāṇī or Her lady friends such as Lalitā and Her serving maids (mañjarīs) like Rūpa and Rati. This is the essence of all instruction in the matter of devotional service."
(Teachings of Lord Caitanya).