Although it does seem that this song is about a significant other cheating, I think it's something a little less drastic. I believe she ends things with him and she's careless about it. He is completely in love with this girl, and though she claims she feels the same, she clearly does not, considering she's ending the relationship. Which is why I think when he sings: "But please don't cry, you liar." She acts as though she's devastated breaking things off, but really, she's just guilty for her lack of love in returning. His words to her are comforting, yet, truthfully harsh: "Please don't cry, you liar." I think he's also numb to her, and that perhaps he seen it coming.
Although it does seem that this song is about a significant other cheating, I think it's something a little less drastic. I believe she ends things with him and she's careless about it. He is completely in love with this girl, and though she claims she feels the same, she clearly does not, considering she's ending the relationship. Which is why I think when he sings: "But please don't cry, you liar." She acts as though she's devastated breaking things off, but really, she's just guilty for her lack of love in returning. His words to her are comforting, yet, truthfully harsh: "Please don't cry, you liar." I think he's also numb to her, and that perhaps he seen it coming.