sa evayam maya te ’dya
yogah proktah puratanah
bhakto ’si me sakha ceti
rahasyam hy etad uttamam
That very ancient science of the relationship with the Supreme is today told by Me to you because you are My devotee as well as My friend and can therefore understand the transcendental mystery of this science.
Commentary by Srila Prabhupada
There are two classes of men, namely the devotee and the demon. The Lord selected Arjuna as the recipient of this great science owing to his being a devotee of the Lord, but for the demon it is not possible to understand this great mysterious science. There are a number of editions of this great book of knowledge. Some of them have commentaries by the devotees, and some of them have commentaries by the demons. Commentation by the devotees is real, whereas that of the demons is useless. Arjuna accepts Sri Krishna as the Supreme Personality of Godhead, and any commentary on the Gita following in the footsteps of Arjuna is real devotional service to the cause of this great science. The demonic, however, do not accept Lord Krishna as He is. Instead they concoct something about Krishna and mislead general readers from the path of Krishna’s instructions. Here is a warning about such misleading paths. One should try to follow the disciplic succession from Arjuna, and thus be benefitted by this great science of Srimad Bhagavad-gita.
Commentary by Sri Visvanatha Cakravarti Thakur
In this verse, the Lord gives the reason for speaking this yoga to Arjuna. You are my devotee (my servant) and my friend. Apart from these two reasons, he gives another reason for speaking to a person favorable to him: it is private or secret.