The destructive power of time on youth and beauty is also a motif in "Sonnet 15." In this sonnet, though, the speaker establishes the context of the beauty of the young man within the grander scheme of life. Life (as the metaphor in the third line suggests) is just a huge stage, and the stars (mentioned in line 4) control the shows or performances upon that stage. Here Shakespeare is expressing a belief in destiny. People may or may not act bravely in their lives, but even the bravest acts of men fade "out of memory" (line 8). Life itself is but an "inconstant stay," a fickle existence of good and bad luck. In other words, one cannot control what happens in life. One has to accept the good with the bad.
Therefore, the speaker values his relationship with the young man. Moreover, he values the beauty of the young man as being one of those random acts of goodness that has somehow entered his own life. Nevertheless, the speaker regrets that this beauty will eventually fade or decay over a period of time (the bad aspect of life that is counteracting the good). So, the speaker feels that he is at war with time. He must preserve the beauty of the young man in his poetry before time eventually steals that beauty away from the young man forever. The poet cannot alter destiny itself, but perhaps he can alter its affects. He can make sure that the young man's beauty does not fade "out of memory."