That time of year thou mayst in me behold
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.
in me thou sees the twilight of such day
as after sunset fadeth in the west;
which by and by black night doth take away,
death's second self that seals up all in rest.
in me thou seest the glowing of such fire
that on the ashes of his youth doth lie,
as the deathbed whereon it must expire,
consumed with that which it was nourished by.
this thou perceiv'st, which makes thy love more strong,
to love that well, which thou must leave ere long.

if we were to remove the final two lines, we could say that this poem was just about

love
old age and dying
relationships
lust