While I agree that one should cherish every moment, love fully and remember that life is indeed fleeting ('Vapor of vapors, all is a vapor' as Ecclesiastes reads in Hebrew) , I do not think that is the moral of Poe's poem.
I think 'Dream Within a Dream' is a deeply sad and tragic poem that causes me to hurt for the writer. I think that it expresses more than just the loss or grief that we all must experience, but rather an unfathomable grief over a deeper eternal loss. An existential pain in a profoundly empty universe.
He fears life is no more substantial than a dream and as impossible to return to. Not a grain of it can he keep from slipping away into the dark abyss.
I think of his poem, 'The Raven, ' where he asks - in effect - if his soul will ever find relief from the great losses of his life. And the raven always replies, 'NEVERMORE! '.
I don't believe that to be true. I believe we will someday be reunited with those we have long ago loved. I believe there is a place of no regrets. I believe there is a place, as author Frederick Buechner puts it, where all that is lost will be found.
At the end of our lives, I believe that will be the deepest yearning of all.