4.3 Because you are my friend Krishna speaks

स एवायं मया तेऽद्य योगः प्रोक्तः पुरातनः
भक्तोऽसि मे सखा चेति रहस्यं ह्येतदुत्तमम्

sa evāyaṁ mayā te ’dya yogaḥ proktaḥ purātanaḥ
bhakto ’si me sakhā cheti rahasyaṁ hyetad uttamam

This same ancient yoga I declare to you today, because you are devoted to me and my friend — and because it is the deepest of secrets.

Explanation

Why Arjuna, and it is not because he is qualified.

Because you are devoted to me, and my friend — bhakto 'si me sakhā ceti.

Two reasons given, and the second is friendship. The deepest of secrets is being handed over on personal grounds, which is the register chapter 9 will use again.

Word by word

saḥ: that
eva: certainly
ayam: this
mayā: by me
te: unto you
adya: today
yogaḥ: the science of Yog
proktaḥ: reveal
purātanaḥ: ancient
bhaktaḥ: devotee
asi: you are
me: my
sakhā: friend
cha: and
iti: therefore
rahasyam: secret
hi: certainly
etat: this
uttamam: supreme