Because I want to? It's not really easy to figure out the true reasons for why one does something (there's a high probability of rationalization), but probable reasons include:

• I like kids, and tend to get along with them well
• it's a nice excuse for being able to play with Legos again
• I like teaching, and finding games for teaching
• A sense of social responsibility: if I can afford to raise kids in a better environment than average, I should do so (see also nykos's comment in this thread)
• My parents, my wife's parents, etc. etc. prefer to have grandkids
• I'm still relatively young, and know that it's better to have kids early than late (it's easier, the kid has a lower chance of having genetic problems, and we have more energy)
• I expect that when I'm older, having kids will be more interesting and useful
• Raising kids is a bit of a challenge

(Yes, some of those reasons could be described as "because it will make me happier")

I plan to because I appear to have a very strong reward circuit around children (especially happy children). I quite literally enjoy a child's smile more than any food I have ever eaten, for example.