I think Ai is faking being happy, because she feels guilty towards Kon.
But as soon as Kaede will reenter the picture, she'll drop everything and say "Sorry" to Kon.
Ai's happiness with Kon is an "ordinary" happiness. Saying "welcome home", working in the day, sleeping together, preparing dinner, lazing on a day off. A very simple happiness. It's suited to Kon, but is it suited to Ai?
Ai's happiness with Kaede is "fireworks" happiness. Going to see the stars, going on random road trips, bungee jumping... a life on the edge, like they're going to die tomorrow so they don't want any regret. A thrilling happiness.
Ai's like a moth to a flame. She's been burnt, will get burned, but she doesn't care. Kon is going to suffer.