why william wordsworth fascinated by the solitary reaper's song?

The poet was in the Scottish highlands when he suddenly heard the maiden reaping alone in the fields and singing to herself. Her tone of sadness or loneliness as well as her melodious voice attracted him to hear her. The poet says that even if he cannot grasp the meaning of the song, he finds the tune touching his heart and lingering in his mind for ever giving him joy despite its melancholy nature. Thus, he became captivated.