He lies to Martha, telling her stories of the beauty and splendour of Gallifrey, saying he won’t take her there because he doesn’t want to go home. Been there, seen it before, travelling is more fun. He keeps lying until Martha backs him into a corner, metaphorically speaking, and compels him to honesty. Then it all comes out.
It feels good to confess, admit the truth. He wants nothing more than to go home, but he can’t, because home isn’t there any more: his world, his people, gone forever. The only thing he doesn’t confess is that it’s his fault.