When someone feels and believes that his life is empty of any meaning, nothing can change his view. And such emptiness seems to be the key for understanding why do we live.
The emptiness of life is a blurred shape which swallows everything meant to fill it: motivations, resolutions, desires, ideals, and so on. All of them become pretty insignificant when enter the emptiness.
For this reasons, many of them can be easily abandoned when the people know better the emptiness of their life. It happens so especially when someone grows older. An older man is more tolerant with the idea that his life can be filled by somebody and something else than by himself, so that he embraces the stance of a simply watcher of what is going on with other people and agrees to be in their number.
Nonetheless, the young men who believe in their full life proves to be only momentary unaware of its emptiness, because their full life is also full of an abnormal agitation which cannot fit to the monotonous course of any life measured by the rhythm of respiration. Therefore, it seems to be a way of salvation from an obscure feeling of the emptiness belonging to their life.
It seems better to not try to fill that emptiness at all and not to deny its existence. Someone who succeeds in doing this can tell that he lives only from himself and thus he demonstrates that his life belongs to him as much as it possible when it is threatened by emptiness.
But only a few can do this, because we always are tempted to search for a full life in other people. They seem to be full of life, as long as we see them filling their own physical appearance with life. Quite irrationally, we betray ourselves by loving them at least until we discover that they have an empty life, too.