Dreams of old friends
There is something profoundly sad about having a dream where one catches up with an old friend, only upon waking remembering that this friend died several years ago.
Paul McCartney mentioned this at least once in a published interview in regard to John Lennon; at the time when I first read it, being all of 16 or so, I could understand it intellectually, but, as is the case with many things, being old enough to have experienced it myself now, it brings another layer of understanding to the sentiment.