Oh, weep for Adonais—he is dead! Wake, melancholy Mother, wake and weep! Yet wherefore? Quench within their burning bed Thy fiery tears, and let thy loud heart keep Like his, a mute and uncomplaining sleep; For he is gone, where all things wise and fair Descend—oh, dream not that the amorous Deep Will yet restore him to the vital air; Death feeds on his mute voice, and laughs at our despair.
Wake up, he says. It's time to mourn, and he wants Urania there to weep for Adonais. Urania here is a symbol for all those that loved Keats. Our speaker wants them to recognize the great loss they now face.
But then he asks: why are we all crying? "Wherefore" may sound like a way to ask where, but it actually means "for what reason?"
The speaker is wondering why they should mourn noisily; he decides it is better to "quench" (hold back) their tears before shedding them. He is telling us not to cry.
So now he tells the mourners to keep quiet, the same way that the dead youth is quiet in the grave.
Is it strange to tell us to mourn, and then to tell us it must remain silent? Maybe, but the speaker tries to explain his change of heart. He says that, since the youth can't make a noise or cry, we should honor him by doing the same.
The speaker also doesn't want us to pretend that the youth can come back, even in our dreams ("dream not that the amorous Deep / Will yet restore him to the vital air"). Death is final, he says. Our tears won't bring him back, so they're pretty much a joke. Bummer.