There are several reasons why a dog may put its butt on a person or other animal. Here are some possible explanations:

1. Marking territory: Dogs have scent glands in their anus, and they may use these glands to mark their territory by rubbing their butt on objects or individuals. This behavior is especially common in male dogs, as it allows them to establish dominance over other males and attract females.
2. Seeking attention: Some dogs may put their butt on a person in an attempt to get attention. This could be because they want to play, go for a walk, or simply be petted.
3. Expressing submission: Dogs are social animals, and they often have a hierarchy within their pack. If a dog puts its butt on another dog or person, it may be a sign of submission, indicating that the dog recognizes the other individual as the leader of the pack.
4. Expressing affection: Dogs are known for their strong bonds with their owners, and some dogs may put their butt on a person as a way of expressing affection or seeking comfort.
5. Seeking comfort: If a dog has had a stressful or anxious experience, it may put its butt on a person or another dog as a way of seeking comfort and reassurance.

It is important to note that every dog is different, and the reasons for this behavior can vary depending on the individual dog and its personality and history. If you are concerned about your dog’s behavior, it is always a good idea to consult with a veterinarian or a professional dog trainer for guidance.