i'm pretty sure this song is about how she fell in love with a boy who was 'crazy' or had some mental illness, and didn't have any faith that their relationship could work because he may have thought that nobody could love a crazy person. "you're a trainwreck but i wouldn't love you if you changed" says that she loves him even if he is crazy, and if he wasn't he wouldn't be himself.
i'm pretty sure this song is about how she fell in love with a boy who was 'crazy' or had some mental illness, and didn't have any faith that their relationship could work because he may have thought that nobody could love a crazy person. "you're a trainwreck but i wouldn't love you if you changed" says that she loves him even if he is crazy, and if he wasn't he wouldn't be himself.