The Lord wants to explain the above-mentioned system of knowledge (knowledge of the phenomenal world and the spirit behind it) because Arjuna is Krsna's confidential devotee and friend. BG 1972 purports

Expressions researched:
"The Lord wants to explain the above-mentioned system of knowledge because Arjuna is Kṛṣṇa's confidential devotee and friend"

Bhagavad-gita As it is

BG Chapters 7 - 12

Complete knowledge includes knowledge of the phenomenal world and the spirit behind it. The source of both of them is transcendental knowledge. The Lord wants to explain the above-mentioned system of knowledge because Arjuna is Kṛṣṇa's confidential devotee and friend.

BG 7.2 (1972), Translation and Purport

I shall now declare unto you in full this knowledge both phenomenal and noumenal, by knowing which there shall remain nothing further to be known.

Complete knowledge includes knowledge of the phenomenal world and the spirit behind it. The source of both of them is transcendental knowledge. The Lord wants to explain the above-mentioned system of knowledge because Arjuna is Kṛṣṇa's confidential devotee and friend. In the beginning of the Fourth Chapter this explanation was given by the Lord, and it is again confirmed here: complete knowledge can be achieved only by the devotee of the Lord directly from the Lord in disciplic succession. Therefore one should be intelligent enough to know the source of all knowledge, who is the cause of all causes and the only object for meditation in all types of yoga practices. When the cause of all causes becomes known, then everything knowable becomes known, and nothing remains unknown. The Vedas say, "yasmin vijñāte sarvam eva vijñatam bhavanti."