Some dogs are people dogs. Some dogs are dog dogs. Some are wary of anyone unfamiliar, human or canine. While there’s definitely a place for socialization, if your dog is highly anxious and reactive in certain situations, it’s best to avoid triggers initially.
For example, if your dog reacts to other dogs on walks, try walking them in a more remote area or early in the morning to avoid negative encounters. Then, work on socialization in a more controlled environment, like with one other familiar dog in a neutral space.
Not all dogs love being around big crowds or surrounded by other canines at a dog park, so follow their lead. If they’re not into it, forcing them into situations they’re uncomfortable with will only elevate their stress levels more.