It can increase his moral energy. It can give him clearer comprehension of other people. It can show a man how to fight without malice, and win without boasting, and lose without bitterness. Fortunately, we can state it, I think, in a single sentence. Let me explain just why it is that this is practical advice.
Gloom is an emotional state, just as mirth is, or joyousness.
And emotion is a form of energy. Indeed, at the human level, emotion is the one form that energy takes. And energy can be put to work. Almost any kind of energy. It can be joy, of course.