If you use your will all the time, then it runs out. You deplete yourself and then, when you really need it, it's not there.
People in the hospital. . . call me the wellest man on the ward. That's what's going to sustain me. I have these energy reserves that I somehow can't seem to deplete.
Exceptionally hard decisions can deplete your energy to the point at which you finally cave in. If you mentally crumble and degenerate into negative thinking, you'll magnify the problem to the point where it can haunt you.