I thought he left because he was afraid that once she knew, Laurel would throw away her pride and put up with Oliver's preference for crusading, yet there was no way to keep the secret without abusing her like Oliver. Tommy struggles with feelings of inferiority. By high school standards of course Tommy's a total loser.
She's five years into the rebound. If the series really means Tommy to be the rebound guy, then they also really mean that Laurel is pretty much all Wuthering Heights of Emo.