A tragic hero always has far to fall, and Doctor Faustus is no exception. A great scholar and polymath, he has an enviable position in life, one for which many people would give their eye teeth. Yet somehow it's not enough. Faustus wants more out of life than he's currently getting. More than anything else, he wants power, and plenty of it; the kind of power that one can only receive from the supernatural. In practical terms, that means the Devil, as God would never grant power to his own creatures that might make them start acting like himself.
So Faustus summons Mephistopheles, one of the Devil's acolytes, and makes a fateful bargain with him. In return for his mortal soul, he will receive twenty-four years of power. Initially, all goes well. Faustus enjoys his increased powers and the renown it brings him. Faustus becomes something of a celebrity and travels all over the world showing off his remarkable abilities.
Yet after a while, it all starts to get rather boring for Faustus. Over time, he gradually becomes dissatisfied, feeling himself to be nothing more than a magician putting on shows for the delight of the rich and powerful. To make matters worse, Faustus cannot ignore the fact that one day all this will be over and that he'll end up going to hell. Crucially, however, he makes no effort to make things right with God, not even in his last few hours on earth.
Eventually, as expected, Faustus is dragged to hell. As with all tragic heroes, a basically good man has become undone by his own hubris. In wanting more power, Faustus was trying to put himself on the same level as God. Such arrogance could only end one way: in damnation.