Faith was deluding herself, and she had to know it, there was little chance of them both staying single. More to the point Tenel Ka wondered if the demon didn't have designs on Sam herself. Not that she was going to burden Faith with that too.
"I think it would be hard. Eventually anyway, which is why I would encourage you to find the right time to tell Sam. I'll help you along the way as much as I can of course but in the end, it has to be you. He deserves to know the truth doesn't he. Otherwise you will grow apart because of your feelings and he won't know why"