My dog wags his tail like crazy when he sees another dog on our walks but every time I let him go up to the other dog he jumps on them and starts a fight. I thought his wagging tail meant he was happy to see the dog and wanted to play.
Dog communication is largely about body language and the more we learn to read our dog’s signals the better we will be able to predict (and prevent) unwanted behavior. When we look at our dog to gauge how he feels it is helpful to have several parts to observe—tail, ears, mouth, forehead and overall stance give us a more complete picture. Tails come in a lot of different shapes and the wagging tail can be misunderstood. Dogs wag their tails when they are excited or aroused, not just when they are happy. Excitement and arousal can go either way depending on the overall situation. Usually the higher up in the air that the base of the tail is raised (above the level of the spine), the more aroused the dog is becoming. A softly waving tail below spine level accompanied by a fluid body movement is more likely a happy dog whereas a tail held high and wagged in short rapid movements is a dog on high alert. If you allow your dog to approach another dog (especially if you are holding them on tight leashes) sparks can easily fly. If you would like to learn more about your dog’s body language, the ASPCA has a great article on the subject: https://www.aspca.org/pet-care/virtual-pet-behaviorist/dog-behavior/canine-body-language.