The saddest thing about growing up is looking at people you've known forever and suddenly realizing that they have changed. A veritable metamorphasis has taken place without you even realizing it (or perhaps not wanting to realize it) that has made them so astonishingly different from who they were that you wonder why you couldn't see it before.

That, and realizing that you cannot look at this new person without sadness and an overwhelming sense of loss for the person you once knew.