Could it be because decision making is hard and you end up telling your partner that you a) don’t care about what they care about b) leave them hanging when they might need help to decide c) they end up having decision fatigue because you don’t have an opinion? Sometimes, a consultation or just a talk about something one tries to make a decision on feels good, doing it all by yourself sucks.
Let’s say you don’t care what hotel she books. She ends up doing all the research on hotels. Presents you with it, pros and cons. You still say you don’t care. Ok, so now the burden of choice is solely on her. You guys arrive, the hotel sucks. Can’t you see that this is frustrating in a different way than if the two of you decided on the hotel together?
I have actually thought it might be misinterpreted because it’s vague in that sentence - no, I don’t mean that the guy thinks the hotel sucks. He still doesn’t care. But his wife thinks it sucks and she is solely responsible for her choice. In a partnership.
Most likely, when confronted with her dislike, he would not be comforting her like “honey, it’s ok, you picked a nice hotel, I don’t think it’s bad at all”, but just be like “look, it doesn’t matter, we’ll just sleep here”. Basically invalidating her feelings and experiences.
The point I am trying to make is: if you are in a committed relationship you sometimes have to care, have an opinion, help with decisions, even if it was something that you usually don’t care about. But saying “I don’t care about the outcome of something that you care about” is definitely neither kind nor loving and devastating in the long run.