We can't help feeling that "The Wild Swans at Coole" is a really sad poem. The speaker picks a gloomy time of year (October) and keeps talking about twilight and how everything has changed (and for the worse, it seems). He talks about his heart being "sore" and exudes a powerful sense of melancholy throughout. In short, this guy haz a sad. Why? There are several good candidates, but the most pressing of these is his wrinkle-phobia, an acute consciousness of his old age.
Even though he contrasts his own weary sadness with the youthful exuberance of the swans, the speaker also implies that there are moments in life where we can momentarily set aside our personal feelings and just enjoy beauty. Ah…
The speaker's sadness is directly linked to his recognition of the reality (and inevitability) of death. Total bummer.