The main cause of Dr. Faustus's downfall is his hubris, or overweening pride. Faustus has a pretty good life, overall, but he's still not satisfied. He wants power; he wants to be renowned the world over as a great genius. But human learning has its limitations. The capacities of the human mind are such as to preclude the kind of power over nature that Faustus craves. So he turns to the dark side for help. He starts dabbling in black magic, making him vulnerable to the overtures of the devil.
Lucifer plays upon Faustus's hubris and intellectual vanity by dispatching his servant Mephastophilis to make him an offer he can't refuse: twenty-four years of service in exchange for his soul. Faustus wavers before entering into the bargain, and the temptation to have everything he's ever wanted is just too great. Although Faustus knows full well that he's playing with fire, that he's placing his very soul in danger, he still goes right ahead and agrees to the diabolical deal, which he signs with his own blood. Ultimately, his overweening pride has got the better of him, and it will lead directly to his eventual downfall.