Bhagavad Gita Verse 3, Chapter 4

sa evaayam mayaa te dya yogah proktah puraatanaha |
bhaktosi me sakhaa cheti rahasyam hyetaduttamam || 3 ||
 
That same ancient yoga I have taught you now, for you are my devotee and my friend. This (yoga) is the ultimate secret.
 
sah : that
eva : same
ayam : this
mayaa : I have
te : to you
adya : now
yogah : yoga
proktah : taught
puraatanaha : ancient
bhaktah : devotee
asi : you are
me : my
sakhaa : friend
cha: and
iti : that is why
rahasyam : secret
hi : because
etat : this
uttamam : ultimate
 
Shri Krishna very affectionately tells Arjuna that he regards him as a devotee and a friend, and finds him fit and capable to receive this supreme knowledge. Arjuna was of course quite happy to receive it, but why was he chosen? Let us investigate this point further.
 
Whoever received this knowledge needed to be capable of communicating it to others. It was not enough that this person was wise. Shri Krishna also needed a capable evangelist, one who had a mix of sattva and rajas, in order to both understand and spread this knowledge. A brahmana is predominantly saattvic by nature, and may not have been able to communicate and establish this knowledge in the world. Therefore Shri Krishna revealed this knowledge to Arjuna, the ultimate kshatriya warrior.
 
Moreover, Arjuna was Shri Krishna's devotee and friend. He had the best rapport with Shri Krishna, as well as profound reverence for him. So much so that Arjuna confided his fears to him in the first chapter, something that a kshatriya would hesitate to do. Arjuna also had the courage to ask any doubts without any fear of reprimand, and in doing so get a clear understanding of the teaching.
 
Many commentators believe that the 108 shlokas starting from shloka 11 of chapter 2 to this shloka comprise the entire message of the Gita. They say that Shri Krishna essentially completed the entire teaching and stopped speaking, having provided the paramparaa as the conclusion. The remainder of the Gita is an exposition of the teachings packed into these 108 shlokas. And it is Arjuna who helps us dig deeper into these teachings with his well-timed and thoughtful questions, like the one we will see in the next shloka.