As a friend, He (Krsna) wanted to inform Arjuna that because Arjuna was the son of Prtha, the sister of His own father Vasudeva, He had agreed to be the charioteer of Arjuna. BG 1972 purports

Expressions researched:
"As a friend, He wanted to inform Arjuna that because Arjuna was the son of Prtha, the sister of His own father Vasudeva, He had agreed to be the charioteer of Arjuna"

Bhagavad-gita As it is

BG Chapters 1 - 6

As a friend, He wanted to inform Arjuna that because Arjuna was the son of Pṛthā, the sister of His own father Vasudeva, He had agreed to be the charioteer of Arjuna. Now what did Kṛṣṇa mean when He told Arjuna to "behold the Kurus"? Did Arjuna want to stop there and not fight? Kṛṣṇa never expected such things from the son of His aunt Pṛthā. The mind of Arjuna was thus predicated by the Lord in friendly joking.

BG 1.25 (1972), Translation and Purport

In the presence of Bhīṣma, Droṇa and all other chieftains of the world, Hṛṣīkeśa, the Lord, said, Just behold, Pārtha, all the Kurus who are assembled here.

As the Supersoul of all living entities, Lord Kṛṣṇa could understand what was going on in the mind of Arjuna. The use of the word Hṛṣīkeśa in this connection indicates that He knew everything. And the word Pārtha, or the son of Kuntī or Pṛthā, is also similarly significant in reference to Arjuna. As a friend, He wanted to inform Arjuna that because Arjuna was the son of Pṛthā, the sister of His own father Vasudeva, He had agreed to be the charioteer of Arjuna. Now what did Kṛṣṇa mean when He told Arjuna to "behold the Kurus"? Did Arjuna want to stop there and not fight? Kṛṣṇa never expected such things from the son of His aunt Pṛthā. The mind of Arjuna was thus predicated by the Lord in friendly joking.