So I think that it's about Linda's ex. Paul can't believe that someone had Linda's love before him but would take it for granted given how important that love is to him.
But I also feel that Paul was especially sensitive to people's love being taken for granted. Given that (and I know not everybody agrees) that he was desperate to keep the Beatles together and John just threw all of it back in his face and basically dumped him for Yoko.
So I think that it's about Linda's ex. Paul can't believe that someone had Linda's love before him but would take it for granted given how important that love is to him. But I also feel that Paul was especially sensitive to people's love being taken for granted. Given that (and I know not everybody agrees) that he was desperate to keep the Beatles together and John just threw all of it back in his face and basically dumped him for Yoko.