I'm not really sure. Many reasons seem to contribute to it. My wife is hard-core Catholic. Her uncle fell apart mentally after losing his only daughter, so having extras is sort of a backup plan. Kids can be fun. Having kids is pretty much the meaning of life.

You may not have intended to come across this way, but I have heard comments similar to yours made to my parents (I have quite a few siblings) and they are offensive.

Think about what it: you're asking about someone's love life. It is no business of yours. So if you're going to ask a question like that, please be respectful. For example, say "If you don't mind my asking, why did you have so many kids?"