The saying "No pain no gain" isnt there without a reason. Weightlifting is painfull and hard and one wouldnt really do it unless it is forced on you (Intelligent motivations aside like trying to attract the opposite gender etc). The same goes for running. I was just out jogging + sprinting and as im typing this i feel sick to my gut after the hard exercise and im concentrating hard at the moment not to throw up.
Why dont we humans or animals have any reward systems for physical workout? Its healthy in all aspects but is preferably avoided.
There are sayings that "resistance make you stronger" but this would only seem to count for "forced" resistance. Noone goes through needless pain for fun.
Is pain actually a protective barrier so to speak - so lifeforms dont kill themselves out of exhaustion? Why isnt there any rewards, only pain - for doing things that are actually beneficial for us?