We Dem Boys!!
Just look at it like he came from hell after having the time of his life and decided to loosen up after taking things more seriously for a while (or getting bored with his job). He did seem excited to go to Hell at the end of DMC2. The only time in the game he seemed enthusiastic about anything and even smiled too.Geeze, that's quite the big change in perspective then. Although that does create a much bigger disconnect between the somber Dante of DMC2 and DMC5 Dante literally hollering "Wahooo! YEEEEEEEAAAAAAAAH~" when we first saw him. That guy is all over the place, like a manic-depressive.
Or maybe he had a Snickers bar.