Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Doctor Faustus deals with some serious—and seriously depressing—stuff. This is, after all, a play about someone who sells his soul over to the devil, and then agonizes about that decision for the rest of the play (and the rest of his life). It's not exactly unicorns and rainbows.
Plus Faustus and Mephistopheles spend a lot of time discussing hell, a place "within the bowels of these elements / Where we are tortured and remain for ever" (2.1.114-115). "Hell hath no limits" (2.1.116), folks, and it's pretty much inevitable that Faustus is going to wind up there. The threat of hell darkens the whole play, making Faustus more than a little mopey at times, no matter how much fun he has with Helen of Troy.
Add to this a parade of the Seven Deadly Sins, Faustus's cruel punishments of courtiers and peasants, and a few murder attempts, and you've got yourself a seriously dark, depressing play, Shmoopers. And we hate to be the bearer of bad news, but the ending is (spoiler alert) not a happy one.
Yet, strangely enough, Doctor Faustus is also funny, at least some of the time. In between all of those discussions about hell and sin, the antics of Wittenberg's peasants—Robin, Dick, Wagner and all their fellow fools—mirror the plot of Faustus and Mephistopheles.
Faustus's servant, Wagner, makes fun of the pretentious Scholars in a seriously silly scene near the beginning. And then, following in Faustus's fiendish footsteps, he takes an unemployed barfly as his "apprentice," a.k.a. servant. Sounds a bit like the Faustus-Mephistopheles relationship, doesn't it?
Plus, there are everyone's favorite fools. When they get hold of Faustus's magic books, town clowns Dick and Robin attempt to wet their whistles with booze rather than solve the world's mysteries, but their magical means are the same as Faustus's.
In these comedic scenes, we get a surprisingly light-hearted take on some seriously dark subject matter. Of course, all this comedy doesn't change the fact that Faustus is headed for hell, which is why the tone of the play is tragicomic, rather than just comic.