Traveling with friends of the opposite sex, why is the relationship suddenly cold after you come back?

I think maybe you are more sensitive. Maybe he has been busy with his work recently, or maybe, as you said, he thinks that if you continue to develop, your's feelings may deviate from his definition of you. He regards you as a good friend, but when he comes back from a trip, he thinks you might like him or something, so he deliberately alienates you. It is also possible that he likes you too, but he is afraid that you will refuse after confession. Don't even have to be friends, just be cold, don't let yourself think too much. These are all possible. Instead of thinking here, ask him clearly.