i believe that while on a mission he got blinded by someone and lost sight in his eye, that could be y there is a scar. then he took a sharingan eye by surgery from a dead member of the uchiha clan, i think that maybe the killing of the uchiha clan and the lose of his eye happened around the same time. so he just got it that way. i no he didn`t get it as a oy because it shows a picture of him, his commarads and his teacher and he doesn`t have the eye.(look in the english manga volume 2 that just came out) in one caption it shows kakashi sleeping with one pic. of him from a long time ago, and another with his new students