[McWrath]

Honestly have no idea.

But seriously, I've never thought about it before.    

Maybe one reason is so i can support and look after my parents when they get old like they've supported me my whole life (Retirement sucks in my country, so if your children don't look after you in old age you are screwed most of the time).

Another i guess more selfish reason is the satisfaction of accomplishing something hard or just being better at something than others.