1) you mostly get this ending if she didn't love him. (Also, if you get this one, they weren't really right for each other, it wouldn't have worked out.)
2) you only get this ending if he loved her, but wasn't prepared to die to be with her. They don't get together when he dies, because he went on to love someone else and have a happy life with them.
3) he's okay with being dead if it means being with her.