Came back different, but no one died. No one did anything irreversible. Two people were together, and then then one of them took a trip and spent a month somewhere else where they couldn't keep in contact. And when they came back they came back surrounded by new people and with a new attitude, maybe even a better and more mature outlook on life. And the person who had stayed behind is just the same.
Maybe that's the root of the trope though. Maybe the root of the trope is always "someone stayed the same and someone grew up." That's why it stings to think about when you read it, because that's the real life version.