It is the lies he's telling her - as he has been, Reissue understands suddenly, her whole life - that really break her heart. He's said that he loves her, after all, but that obviously isn't true. He cannot love an orogeny, and that is what she is. Furthermore, he cannot be an orogeny's father, and that is why he constantly demands she be something other than what she is.

The Obelisk Gate