Just because you feel like shit and your head is saying "this sucks ass", there's also the endorphins being released, the feeling of achieving something, of overcoming your own limitations and stuff.

I mean it's not for me but I can see how in hindsight (it's always in hindsight, never in the moment) it'll be considered fun. Or at the very least a story to share with friends or the internet, which is also fun.