I don't know that that's true. I think he was a glory hound who wanted to lead victorious armies. But more than that, he didn't want to lose a battle, which paralyzed him. Even with Lee's battle plan in his hand, at Antietam, he was timid.
On the plus side, he did build the Army of the Potomac. The problem was that once he had them, he was afraid to do anything with them. The Peninsula Campaign was a brilliant idea, but once he landed the army, he delayed and delayed getting them moving until Lee was able to get his forces organized.
That is about the time that Lincoln said to McClellan, “If you are not going to use the Army, do you mind if I borrow it?”