In "The Wild Swans at Coole," the speaker sees fifty-nine swans swimming on the water and reflects that he first saw and counted the swans here nineteen years ago. Reflecting upon the passage of time, he says that his heart is sore. Everything has changed in his life since the first time he saw the swans.
The speaker is not explicit about the nature of the change, but he gives the reader various hints. The poem is set in October, at twilight, meaning that it is near the end of the year and the day. The speaker does not say how old he is, but if he was a young man* when he saw the swans, he must be middle-aged now. There is a wistful, melancholy tone in the poem, which suggests that things have changed for the worse.
The speaker also remarks that the swans' "hearts have not grown old." This suggests that his heart has grown old, and he is alone. The swans, unlike him, are in pairs, "lover by lover." If there are fifty-nine of them, then at least one must be alone, but the speaker does not focus on this, emphasizing only the way in which these "brilliant creatures," unlike him, seem to be untouched by the passage of time and therefore remind him of his own age.
*The sex of the speaker is not mentioned, but the poet is male and seems to identify closely with the speaker.