Your ex-lover cheated on you with Zach's ex-lover. You were hurt and hurt..That's why you and Zach decided to take revenge on them by becoming a fake couple, while keeping the secret so that your former halves wouldn't find out that you were hanging out together. You posted photos together to make your exes jealous, but it didn't seem like it was enough, and Zach decided to take a more intimate photo to definitely attract their attention. He already knew what kind of picture he was going to take with you.