The speaker in 'Piano' remembers his childhood while listening to a woman sing to him passionately at dusk. This woman in all probability is his lover, and the speaker is a grown man, who considers it to be his duty to listen to the woman who loves him. Moreover it is expected as a man he should have better control of his emotions. However the insidious mastery of song betrays him, and he starts crying like a child, instead of behaving as a grown man should.
Because he is sad