Disclaimer: Harry Potter isn't mine.
The problem with being intelligent is that eventually you realize your intelligence. Then you take a look around, and begin to wonder, "Why are these idiots in charge? I would do so much better!" Then, if you're of a particular turn of mind, you begin to think of how to put yourself in charge; for the greater good, naturally. I realized how bright I was on the first day of Hogwarts, when everybody, including the teacher, stared at me when the transfiguration right on the first try. That set the pattern, of both inadvertent achievement and effortless accomplishment. By fourth year, there was nothing they tried to teach me that I didn't already know. I was bored, and I had found a way into the Restricted Section of the library. I read about Dark Arts; Blood magic, Necromancy and so much more... I had found power, and it was addictive.