Say for example you were doing your damnedest to save humanity and the planet from a dark impending fate, then something reminded you of how nasty and selfish humans can be and you wondered why you'd bothered to give up your life for the past few years.
That's the feeling you'd get as the dust swirled up your nostrils and down your throat, but then, as you ploughed on through, the fresh air and beaming light reminded you it was all worthwhile.