i think this song is about a guy who has a girl friend but hes in love with someone else and he's realising that she is his 'only one' and that he doesnt love his girlfriend anymore. "what do you say when your hearts not in it" this is saying that he hasnt got the feelings for his girlfriend that he has for this other girl and hes asking himself what hes going to do and how he's going to tell his girlfriend he doesnt like her anymore and that hes in love with the other girl and always has been.
i think this song is about a guy who has a girl friend but hes in love with someone else and he's realising that she is his 'only one' and that he doesnt love his girlfriend anymore. "what do you say when your hearts not in it" this is saying that he hasnt got the feelings for his girlfriend that he has for this other girl and hes asking himself what hes going to do and how he's going to tell his girlfriend he doesnt like her anymore and that hes in love with the other girl and always has been.