tone is unabashedly pained and sad. He is expressing a deep seeded worry--a pathological fear of not living and experiencing all he longs to taste in life--Keats had experienced much loss as a boy--this sonnet is a reminder of sorts that, although he is young (when it was written), he must always be waiting for whatever happiness graces him at the moment to be spoiled by despair coming down the tracks...achingly beautiful, his life and plight--don't you agree?? I had written a paper about how this sonnet is a portention of sorts, as it almost eerily seems to predict his own untimely death...all of his work possesses that otherworldly quality--it's just one of the things that helps pronounce his genius.