Faustus is super-smart. So smart that he can best any one of his academic colleagues in debate, so smart that he becomes arrogant, "swoll'n with cunning, of a self-conceit" (Prologue.19). And that's where all the trouble begins. See, Faustus thinks he knows better than the thousand years'-worth of scholars who have gone before him and been content to devote their lives to philosophy, medicine, theology, or the law. He thinks those disciplines are totally useless.
Why? Because he thinks they can't do anything for him, or at least, nothing like what magic can do. And magic can do a lot. Which is good because in addition to being arrogant, Faustus is also power-hungry. He makes it clear that he longs to learn magic because "a sound magician is a demigod" (1.1.60). Does that sound eerily familiar? It should. Lucifer, himself, fell from heaven because in his pride he, too, wanted to be a god.
You'd think Faustus would learn from old Lucy's mistake, huh?
That said, maybe Faustus could halfway redeem himself if he used his magic to do something worthwhile, like, say, saving lives or saving souls. But instead, he uses it mainly to idly amuse himself and, most despicably, to play mean tricks on peasants and courtiers like poor Benvolio.
What Faustus chooses to do with his powers after he sells his soul to gain them is a bit anticlimactic considering that he had planned to "resolve me of all ambiguities," learn "strange philosophy," and "the secrets of all foreign kings," and even to become the king of his land (1.1.78, 84, 85). We don't know about you, but Shmoop really wanted to be resolved of all ambiguities. That sounds awesome.
But no sooner has Faustus gained his awesome powers than his lofty ambitions fade into something much more, well, boring. Why he loses those ambitions is a bit of a mystery. It might be an example of the old maxim that "absolute power corrupts absolutely," except that in this case, absolute power appears to corrupt not just the man who possesses it, but also the goals that prompted him to get that power in the first place. This whole demi-god thing is a messy business.
So he's proud, he's arrogant, and he's sometimes more than a little mean-spirited. Why in the world are we reading about this guy, then? Sure, his pact with the devil seems to have been motivated largely by a desire for wealth and power. But the text also suggests that something else might be going on. Maybe Faust thought he never had a chance at heaven to begin with.
Think about it. As Faustus contemplates theology, Mephistopheles guides his reading of the Bible so that he concludes that "we must sin / And so consequently die"—i.e., go to hell (1.1.43-44). Throughout the play, Faustus mopes over this question, always concluding (with the help of that Bad Angel on his shoulder), that he's got no shot at salvation. He's doomed from the get-go, or at least so he's led to believe.
Unfortunately, though, Faustus might not be as smart as he thinks. He never quite manages to think all the way through the question of whether or not he has a chance at a ticket to heaven. He's always distracted by his devils bringing some new delight.
In the end, Faustus "didst love the world"—in other words, material things—more than spiritual things (5.2.101). His pride and desire for power prompt him to sign his soul over to the devil. Once that's done, his inability to let go of all the awesome stuff his devils bring him prevents him from ever repenting. He is, plainly, weak. For that reason, we pity Faustus. He comes so close to heaven so many times, even as he misses it by a mile.