The reason why the old man said he cannot ride a dog is as follows:

From a moral perspective: Dogs are human companions and good friends , we should coexist peacefully with them, rather than treating them as toys or tools. The behavior of riding a dog violates the respect and protection of animals and is a serious moral issue.

From a safety perspective: The body structure of dogs is very different from that of humans. Our bones are stronger than dogs and are more suitable for bearing weight. Riding on a dog can easily lead to overexertion, injury or even death. In addition, riding dogs in public places will cause walking and traffic problems to other people, causing danger and injury.