Nor let us weep that our delight is fled Far from these carrion kites that scream below; He wakes or sleeps with the enduring dead; Thou canst not soar where he is sitting now. Dust to the dust! but the pure spirit shall flow Back to the burning fountain whence it came, A portion of the Eternal, which must glow Through time and change, unquenchably the same, Whilst thy cold embers choke the sordid hearth of shame.
Now the speaker is telling us to stop weeping. But… didn't he just spend thirty-eight stanza basically telling us why we should mourn? We guess he's changed his mind.
He says that "our delight" (Adonais-Keats) has gone far away from the "carrion kites" (corpse-eating birds) on earth. He is now in a more eternal place, a place that endures. We can't visit him in Heaven. Only "pure spirits" get to hang out up there.
Shelley uses the imagery of a burning fountain to symbolize eternal life. Keats is now part of that fountain.
It never changes. Therefore, Keats is always part of this energy. That's Shelley's version of heaven.
Meanwhile, here on earth, "cold embers" are choking us with shame. He's glad that Keats is free of all that.
That's why he thinks we shouldn't mourn. Keats is in a better place. So… yay?