Let me start by saying, I know the official order of the games is Devil May Cry 3, Devil May Cry, The Animated Series, Devil May Cry 4, and and Devil May Cry 2. Don't flame me for what I'm about to say.
Also, let me say that I am personally hoping that "Dante" in DMC2 is actually Vergil. He's a cool character that I'd like to bring back. If what I am about to say ends up as true (it won't,) then Vergil isn't coming back, and that makes me a sad Devil Hunter.
But, here's a theory:
Here's my logic:
The events up to DMC1 happen as normal, but then the events of Goikeda's second novel happen, which sends Dante is a depressed state, altering his personality for DMC2. Then, some unknown event happens in which Dante returns to the Human World. He's still a bit reserved, but has his "spark" back, even if he's still a little on the somber side. His new friends, Patty and Morrison, help bring him out of his rut, and he's practically back to his cocky self for DMC4.
Make sense? Or is it too farfetched?