Few feelings are as hollowing as a deep sense of uselessness.
That feeling of uselessness, while often initially combatted with an inefficient and poorly choreographed attempt at egotistic self edification, typically evolves into a debilitating morass of self pity.
Self-pity makes others dislike us, but uselessness makes us dislike ourselves. There is no loneliness like sitting around talking shit about yourself to yourself.
We overcome uselessness with usefulness. By finding a purpose, and then performing a function toward that purpose. Whether evolutionary or spiritual in nature, we are not happy when we are not contributing.
However, in order to contribute we must be capable. For me, fitness is the restoration and maintenance of the capability to be useful. And when I am not actively performing a useful function, I am keeping myself prepared for one.