Why do we do it?

Today I have a question for all mothers out there.  Tell me why do we do it?  

Yesterday I sat down with Jake to fill out a paper to introduce him to his new kindergarten class.  There was a question that asked what do your parents do?  His response was "my dad works and my mom cleans."  I didn't write that I "helped" him change it to my mom stays home with us. 

And today my nephew was over playing with the boys and he asked me why I don't work.  I said I like to stay home with the boys.  He said "Do you stay home to ground them?"

Really? is that really why I stay home with my children.  I'll admit I do clean, and i do ground them.  But I play with them too.  I teach them too don't I?   So why do we do it?   Even if we do work, why do we raise these little bundles of joy and why do we keep having more?  That's my question today.  Why do we really do this.