As friends, both of them (Krsna and Arjuna) were on the same level, but one of them voluntarily became a student of the other. Krsna was smiling because a friend had chosen to become a disciple. BG 1972 purports

Expressions researched:
"As friends, both of them were on the same level, but one of them voluntarily became a student of the other. Kṛṣṇa was smiling because a friend had chosen to become a disciple"

Bhagavad-gita As it is

BG Chapters 1 - 6

The talk was going on between intimate friends, namely the Hṛṣīkeśa and the Guḍākeśa. As friends, both of them were on the same level, but one of them voluntarily became a student of the other. Kṛṣṇa was smiling because a friend had chosen to become a disciple.

BG 2.10 (1972), Translation and Purport

O descendant of Bharata, at that time Kṛṣṇa, smiling, in the midst of both the armies, spoke the following words to the grief-stricken Arjuna.

The talk was going on between intimate friends, namely the Hṛṣīkeśa and the Guḍākeśa. As friends, both of them were on the same level, but one of them voluntarily became a student of the other. Kṛṣṇa was smiling because a friend had chosen to become a disciple. As Lord of all, He is always in the superior position as the master of everyone, and yet the Lord accepts one who wishes to be a friend, a son, a lover or a devotee, or who wants Him in such a role. But when He was accepted as the master, He at once assumed the role and talked with the disciple like the master—with gravity, as it is required. It appears that the talk between the master and the disciple was openly exchanged in the presence of both armies so that all were benefitted. So the talks of Bhagavad-gītā are not for any particular person, society, or community, but they are for all, and friends or enemies are equally entitled to hear them.