[zer0gravity]

Actually our physiology, may constrain us to this type of behaviour, throught the inner reward system that releases certain dopamine and other "feel good" neurotransmitters only after a certain level of effort / stress has beed detected, that is after high levels of cortisol, which is connected to stress .

This is of course a very simplistic description and by no means explains the result but it may play an important role.

This is a mechanism evolved since our existence in the wilderness and may have very different/negative results in our modern lives if not filtered through higher level mental functions. But anyway giving the fact that it's such a low level mechanism is hard to control it totally..