If I don't regularly exercise, I'll get burned out very quickly on some task I would have normally found fun previously. E.g. some highly-intense game like a competitive multiplayer shooter, or even a difficult programming challenge. Even interacting with other people can suck in this mode.
If I am regularly exercising (I.e. 30 minutes of cardio every day or better), I can carry through marathon sessions of gaming or coding with a smile on my face the entire time. My social tolerance levels are infinitely higher than without.
I still don't fully understand all of the biology behind this, but it seems that regular exercise is a great way to pay-forward the debt that is incurred when you stimulate your dopamine receptors. Maybe there is some relationship between exercise/sleep/dopamine/serotonin, because I find that regular exercise yields substantially deeper sleep, and when I wake up from this I feel like I want to engage in challenging tasks.