The ending is perfect in many ways. If it ended up being too happy and easy with everything resolved, then it would have been a bit too much fan service and unrealistic (ha! I say that about a drama that is about an alien lol), it's just that it leaves unanswered questions. And the biggest one is how long can they really be happy together when we know the issue of her aging while he doesn't is still up in the air? I know not everything can end perfect, but it kind of makes it a bit sad doesn't it?