My musings: Why did Krishna became the charioteer of Arjun?

Krishna could have become charioteer of Yudhishthir, as he was the eldest (known) brother of Pandavas, and he was not very much willing to fight. If he said anything, Arjuna would have done that, without any questions.

Bheem was exceptionally charged and angry at Kauravas. He would have become emotionally stable and probably beaten the crap out of every single Kaurava if Krishna was with him.

Similarly, Nakul and Sahdev were usually in the background. With Krishna's support and Gita Gyan, they could have come to the foreground and become a formidable force.

My theory is this:

Yudhishthir was balanced minded, and almost observed everything with a calm mind (except in Mayasabha, we don't know what came upon him). While he had agreed to lead the war, probably he didn't require anyone to get him more clarity.

Bheem was better off as an angry man. He had muscle power, and one requires adrenaline rush to invoke all of the muscle power. Adrenaline rush never comes with a calm mind.

Nakul and Sahadev had their strength in being in background, and war requires a few forces to work behind the scenes. Sahadev was a master strategist, and such people already have enough clarity. With their thought process, they had a strong motivation to fight this war with the Kauravas.

Arjun was one of the best in archery, and he had a lot of doubts. He was a poetic guy, a lover boy who would always see humanity and logic as trade offs rather than complimentary thought processes. He would be angry, but would not have the clarity to fire arrows at his grandfather Bhishm or teacher Drona (the humane angle of relations etc taking over the logical duty of fighting in the war), etc. Probably that's why he needed Krishna's guidance more.