The pain, the bleakness of life, the unbearably weight of reality itself pressing down upon him, crushing him beneath it's weight as his sole anchor in life, his love, is brutually torn away, taking with her his happiness, his joy, leaving him as a sad, hollow shell of his former self.
His despair and misery eating away at him till all that is left is his miserable existance. Till finally he can stand it no more, with a shot, his misery, his pain, his anguish over the one he lost, simply disappear.