In a world full of angst, Deadpool is probably just about the most upbeat hero in the Marvel roster, which basically also means the major superhero universe - given Marvel is essentially DC's more cheery younger brother.
With the vast majority of heroes having apparently never left their cynical teenage phase, a sassy, almost psychopathically cheerful character is a breath of fresh air in an otherwise brooding landscape.
However, Deadpool is also a three-dimensional character, and as such has his fair share of misery and misfortune. The beauty of a character like Deadpool isn't that there's no sad moments, but more that there tend to be more funny and happy moments, making room for a comic audience who don't want to feel miserable after reading.
That said, having an almost permanently jolly hero also makes sad moments feel all the worse when they happen, because Deadpool isn't the type to throw himself into moping and brooding at the drop of a hat.
From being betrayed by the people he loved most to being kicked out of his own Christmas party, there are a whole heap of comics that prove even light-hearted comics can provide the emotional equivalent to a kick in the balls.