By execution of devotional service under the guidance of the spiritual master, one becomes free from all material attachment, attains steadiness in self-realization, and acquires a taste for hearing about the Absolute Personality of Godhead, Śrī Kṛṣṇa. This taste leads one further forward to attachment for Kṛṣṇa consciousness, which is matured in bhāva, or the preliminary stage of transcendental love of God. Real love for God is called prema, the highest perfectional stage of life." In the prema stage there is constant engagement in the transcendental loving service of the Lord. So, by the slow process of devotional service, under the guidance of the bona fide spiritual master, one can attain the highest stage, being freed from all material attachment, from the fearfulness of one's individual spiritual personality, and from the frustrations that result in void philosophy. Then one can ultimately attain to the abode of the Supreme Lord.
(Bhagavad-Gita----4:10---purport).

This love of God is now in a dormant state in everyone's heart. And, there, love of God is manifested in different ways, but it is contaminated by material association. Now the heart has to be purified of the material association, and that dormant, natural love for Kṛṣṇa has to be revived. That is the whole process.
To practice the regulative principles of bhakti-yoga one should, under the guidance of an expert spiritual master, follow certain principles: one should rise early in the morning, take bath, enter the temple and offer prayers and chant Hare Kṛṣṇa, then collect flowers to offer to the Deity, cook foodstuffs to offer to the Deity, take prasādam, and so on. There are various rules and regulations which one should follow. And one should constantly hear Bhagavad-gītā and Śrīmad-Bhāgavatam from pure devotees. This practice can help anyone rise to the level of love of God, and then he is sure of his progress into the spiritual kingdom of God. This practice of bhakti-yoga, under the rules and regulations, with the direction of a spiritual master, will surely bring one to the stage of love of God.
(Bhagavad-Gita---12:9---purport).

The ultimate result of devotional service is to develop genuine love for the Supreme Personality. Love is a word which is often used in relation with man and woman. And love is the only word that can be properly used to indicate the relation between Lord Kṛṣṇa and the living entities. The living entities are mentioned as prakṛti in the Bhagavad-gītā, and in Sanskrit prakṛti is a feminine object. The Lord is always described as the parama-puruṣa, or the supreme male personality. Thus the affection between the Lord and the living entities is something like that between the male and the female. Therefore the term love of Godhead is quite appropriate.
(Srimad Bhagavatam---1:7:7---purport).

Lord Kṛṣṇa instructed Arjuna in the Bhagavad-gītā because He found Arjuna to be a devotee and friend. Without such qualifications, one cannot enter into the mystery of the Bhagavad-gītā Therefore, one cannot understand the Personality of Godhead unless one becomes a devotee and discharges devotional service. This mystery is love of Godhead. Therein lies the main qualification for knowing the mystery of the Personality of Godhead. And to attain the stage of transcendental love of Godhead, regulative principles of devotional service must be followed. The regulative principles are called vidhi-bhakti, or the devotional service of the Lord, and they can be practiced by a neophyte with his present senses. Such regulative principles are mainly based on hearing and chanting of the glories of the Lord.
(Srimad Bhagavatam---2:9:31---purport).