"You're not the person I thought you were"
I'm as guilty of this as the next person, and could tell you at very great, very raw & painful length how it happens, but that's not my point. Don't try to make them something they're not; don't try to make them what you want them to be & get hurt when it doesn't work out. Take someone as they are, and love them for that.
p.s I can't even remember what the miscellaneous beverage was this time. Still wasn't coffee though