Because if there's one thing this place has taught Jack, it's to grab those second chances with both hands and never let go.
Even if - it's weird, but... He remembers that stuff with Ianto when they first turned up, and yet... He hasn't thought of Ianto all day. Because he's got Kitty now - yet he can't remember when all of that changed. When his affections shifted from Ianto, to Kitty.