This song seems to be about a couple who split up during the summer and are expressing how they feel about the relationship. The girl's bit is the first verse: she cheated on him but it didn't mean anything compared to what he feels ("The grass stains on my knees don't mean a thing"). The second verse is the guy telling the girl he can't lie to her, and that he's missed her and he forgives her no matter what: "If you could slit my throat, with my one last gasping breath I'd apologize for bleeding on your shirt." The chorus is saying they miss each other. They should hate each other but don't.
WarrenHBW - Norwich, England