Disclaimer: The characters of House, M.D. aren't my creations and I therefore make not a penny from writing about them. Woe is me.
When Cameron asks you, you lie to her. You tell her she's hallucinating, that she only thinks she sees that softness, that warmth in the soul of Greg House. It isn't there. There's no fuzzy underbelly, no sweetness stored away inside the stone, and (at least this part is true) she may as well stop trying to find it.
You are lying, lying, lying. Cameron doesn't know the half of it, the way Greg would wake up in the morning and watch you waking up beside him. The feel of his hand in your hair, steady and gentle as he held you. The way he would, on occasion, simply curl up in your arms and let you soothe him. She doesn't know all that. She guesses at it, because she has seen one part of the truth, the irreparable wound in his heart. You learned to live with that, to ease that pain in whatever ways you could because you knew that it would never really go away.
And that's why you lie to Cameron, that fool who wants to play Beauty and the Beast with him. She thinks that if it's there she can heal it. You watch her turn around and leave, like a child who's just been told that Santa Claus is dead and Christmas has been cancelled.
There's a way in which you feel for her, and a way in which you've done Greg an injustice, but you can't tell her the truth. If she knows, and if he gives her the chance, this girl will take him apart. She will pick, pull, cut away at him, trying to separate the velvet from the tangle of razor wire, unwilling to believe that it can't be done. She wants to destroy what he is so that he becomes what she thinks he ought to be.
You're really not sure what that is, but it is not love, and the sooner she gets over it, the better.