You're being one now. You're telling me the truth. [More truth than he wants to hear all at once, even!
The but makes him tense and prepare for something bad. When something bad doesn't come, he waits for her to continue for a moment before concluding that that's it, that was the bad thing.]
Kitty, Jack is your friend and Reid is... [uh?] Reid. Yes, keep them, of course. [A little insecurity does creep into his voice in spite of the lack of badness. She's extremely attracted to Jack, some part of her is still in love with Reid, and she has known both of them longer than she's known him.] But try to always come back to me.