In this poem the speaker expresses a powerful longing to retreat from modern life and the younger generation, among which, as an ageing man, he feels he has no place. He thus imagines a journey to the ancient city of Byzantium (modern-day Istanbul) which functions as a symbol of spiritual renewal.
The first stanza is taken up with a depiction of youth and its pleasures - the young/in one another's arms' - and contains images of fertility among both human beings and animals - 'fish,flesh, or fowl'. However, the younger generation, with all their careless joys, ignore the old:
Caught in that sensual music all neglect
Monuments of unageing intellect.
This concluding couplet of the first stanza voices the poet's complaint that younger people are too caught up in their own pleasures to have any time for the old. In sharp contrast to the lush portrayal of youth, the old are starkly imaged as 'monuments', which has a rather de-humanizing effect. The implication is that the old have become dry, devoid of those emotions which fire the blood of youth. However, this is only the outward appearance: the poet's mind, at least, is as active as ever.
Yeats's complaint is that the modern world focuses too much on the young and is given over to the enjoyment of feeling and instinct rather than concentrating on the world of thought and spirit, as represented by the 'unageing intellect' of the older and wiser generation. From his perspective, people in the modern world are unthinkingly caught up in the perpetual cycle of birth and death.This is why he longs to get away.