Expert Answers
amarang9 eNotes educator| Certified Educator

In Sonnet 15, the speaker is considering the transitory, temporary nature of life. He recognizes that every thing that lives ("grows") reaches a prime in his/her/its life and that peak moment lasts only a brief time. The world is compared to a stage where the stars (astrologically) influence or direct the actions of the players (humans). Plants, like men, "increase" (grow) but when they reach their height, they begin to decrease and wear out. The speaker imagines the height of his companion's youth ("you" the addressee in the sonnet) in spite of the limited (inconstant) time on earth. Moving toward the end of a life, time and decay debate on how to change one's life from youth to old age and then to death: 

Then the conceit of this inconstant stay

Sets you most rich in youth before my sight,

Where wasteful time debateth with decay

To change your day of youth to sullied night; (9-12) 

In the end, the speaker says that we "all" are at war with time itself. For the love of the sonnet's addressee ("you"), the speaker says that as time takes "your" youth, he will "engraft you anew." This is a common theme in some of Shakespeare's sonnets: that although life is fleeting, he can immortalize those he loves in the sonnet itself (engraft meaning to write).