Ah, woe is me! Winter is come and gone, But grief returns with the revolving year; The airs and streams renew their joyous tone; The ants, the bees, the swallows reappear; Fresh leaves and flowers deck the dead Seasons' bier; The amorous birds now pair in every brake, And build their mossy homes in field and brere; And the green lizard, and the golden snake, Like unimprison'd flames, out of their trance awake.
The speaker's grief hasn't gone away, despite the change in seasons. His grief "returns with the revolving year."
The rest of the world continues on, though. He describes nature as participating joyfully in the Spring, where flowers "deck the dead Seasons' bier" or, cover the metaphorical coffin of winter.
Birds are pairing up and making nests, lizards and snakes are waking up out of their "trance" of Winter, and Spring is officially here.
So, why such a lively stanza in an otherwise mournful poem? One answer is to emphasize that Adonais-Keats is the opposite of these creatures. He's dead, and the world has decided to keep living, despite what we saw in earlier stanzas.
The poem is continuing to become more about the natural world and less about Greek goddesses and personified seasons.