I like to say that, in being a parent, I'm less happy on an "instantaneous" basis, but more happy overall; more fulfilled. I expect, to some extent, it has something in common with volunteering at a charity. It's not the actual act of doing the work that makes you happy, it's the fact that you're doing the work for a reason.

Would I love to sit around and play video games in the large amounts of free time I would have if I didn't have a family? Sure. But that instantaneous gratification does not compare to the more "overall" happiness of having a family. For me. Clearly, it's different for everyone.