This poem is comparing seasons and our life. He is especially focusing on towards the end of our lives. His description of the autumn of our life is very dreary. He portrays like this:
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang.
This description makes me fear about my future, when my life get into that season. I think Shakespeare was try to convey that we should love our youth through this poem.