Tim may not have a major in psychology like Lucy, but he had gone through a traumatizing childhood and more than one round at war, not to mention the many years in patrol. He was able to see, first hand, the crazy things people could normalize in their heads and actually get used to. Some of them, although horrible, get stuck with you to the point where you can never fully go back. He was now learning that the same could happen to not-at-all-horrible stuff as well. Like kissing Lucy Chen, for example.