Why does Erik get so mad at Christine?

Erik falls in love with Christine when he meets her. For a while, he pretends to be the Spirit of Music that her father once told her about, and he trains her to become a great singer. When he is unable to make her love him, though, he becomes insane. He kidnaps her and eventually threatens to blow up a quarter of the town with all of the gunpowder he has hidden beneath the opera house. In the end, though, his heart is softened by her promise to love him, and he lets Christine and Raoul go free to marry each other.