Could someone please explain to me why babies fight sleep?

Daniel sleeps very well at night (8pm to 6am) however, during the day he fights the inevitable. He eventually gets to sleep, after much rocking and hugging and has a twenty minute "brain switch off".

I certainly don't fight it. Just wondering why babies do. Is it because they'll miss all the exciting things going on? Does Daniel think I party on down when he asleep???

"Shhhh, Jason, don't let Daniel hear you playing with his Fisher Price toys!!"