This song is about IU dating a boy that can't seem to let go of his ex-girlfriend that supposedly broke up with him and left him coldly. Even so, the boy was still attached to the girl. He would willingly pick up a call from his ex-girlfriend, constantly day dreaming and thinking about her while him and IU were talking with each other. IU, who noticed his actions, was very annoyed by it. She would always question in her mind that, if they ended their relationship really harshly, why bother calling all day long. She, even though had never met the ex-girlfriend, despised the girl a lot. She was overall jealous at the relationship they were having currently and IU started to think what's so great about the girl that his boyfriend would never seem to let go of her and forget the past. That's all, my favourite song from IU must be explained. Sankyu.