Most people think that when a dog's tail wags, it indicates that they are in a good mood . The tail is a communication tool for dogs, but you have to pay attention to the position of the tail in order to get insight on a dog's mood. The natural position of a dog's tail varies according to the breed, but when the tail is in this state, it shows that the dog is relaxed. If the dog is anxious, it's tail will be lower than normal. If the dog is feeling aggressive, the tail may be at a vertical angle. And, to answer your question, a wagging tail is generally associated with excitement. Studies also indicate that if the tail wags more to the right, the dog is happier. If it wags more to the left, the dog may be upset about something.
Therefore, do not assume that just because a dog's tail is wagging that it is okay to pet him or her .