Kakashi becomes Jounin. His team give him gifts. Uchiha Obito doesn't. Uchiha Obito gets crushed by rocks. Kakashi's right eye is scratched (how he got the scar). Kakashi can't see in eye. Obito gives his left Sharingan eye as gift to Kakashi. The girl in Kakashi's team is medical ninja. She puts obito's eye into Kakashi's eyesocket thing.

There.