The tragedy that befalls Doctor Faustus is ultimately caused by what the ancient Greeks called hubris, or overweening pride. Faustus is profoundly dissatisfied with life. He's an exceptionally intelligent man, a polymath with a genuine hunger for knowledge. Yet he feels frustrated with the limits of mere human knowledge; he wants to do more; he wants to have magic powers. This gaping void at the heart of Faustus's intellectual life makes him vulnerable to corruption by the devil. So desperate is Faustus to practice the black arts that he agrees to sell his immortal soul to Lucifer in exchange for twenty-four years of Mephistopheles's service.
Faustus's pride will simply not allow him to accept that he's just a human being, with all the necessary limitations that that involves. By engaging in sorcery he hopes to turn himself into some kind of super-human, almost a god. And though he knows what he's doing is fundamentally wrong, he still does it anyway. His overweening pride, his hubris, is too strong for him to turn back and repent of his sins. A man of great intelligence has overreached himself, and the consequences for his immortal soul are catastrophic in the extreme.