The Mahabharata - pg. 226

"Dhananjaya, why did you choose thus unwisely, preferring me alone and unarmed to my full equipped and heroic forces?" asked Krishna to Arjuna with a smile, when they were alone. Arjuna answered: "My ambition is to achieve glory even like yours. You have the power and prowess to all the princes of the land and their hordes in battle single-handed. I too feel I can do it. So, I desire that I should win the battle with you driving my chariot unarmed. I have desired this for long and you have today fulfilled my wish."
Vasudeva smiled again and pronounced this benediction: "Are you trying to compete with me? May you succeed," for he was pleased with Arjuna's decision.
This is the sacred story of how Krishna became Partha's charioteer.