Why does my dog all of a sudden hate the bus?

Your dog has developed a negative association with the bus ride that was not previously present. Even though you may not have noticed it, something changed that caused him to feel stress, pain, or discomfort at some point, and he is now associating the bus as a "bad" place. If you are worried about health issues, by all means consult your veterinarian. Otherwise, I would advise that you try to help re-condition him to make this a more positive experience. Try bringing a toy on the bus that he doesn't normally get, but that makes him happy. You can bring treats or pieces of kibble and reward him for good behavior. You may be able to help him get past his anxiety and enjoy his little naps again!

It's hard to know why. Perhaps there was a sound or a smell that disturbed him. I had a similar experience with my dog. The first time she rode an elevator she was totally fine, and as far as I could tell, nothing happened that upset her. But on subsequent rides, she was nervous. Who knows?

Probably the best approach is to ignore his changed behavior. If you coddle him, or even give him a "pep talk," he might view that as "proof" that there is something to be afraid of. If it was possible to do gradual desensitization, that would be better, but I can't think of a logistical way for you to do that. It's not like you have the option of walking up to the bus, giving him a treat and praising him before he has a chance to become afraid, then walking away, then repeating that, getting closer and closer. You have to go to work, and you have to actually ride the bus to get there.