I agree with several points made already especially about decentralization. None of my 4 care for Boxers- no clue why, but it was suggested at one incident at the vets office with sweet Galina barking at one, that it could just be the change in the muzzle in what my dogs are use to seeing most of the time. ( long muzzle verses short). I also saw a show about a racing greyhound that was worried about all over dogs, but obviously fine- even more outgoing when returned to the track for a visit with her new owner. It was obvious the dog only recognized another dog if the one looked similar to themselves. The racing greyhound had an excuse- it had lived its whole life with only greyhounds and people. With other dogs, this can be socialization issues.
Due to the fact two of mine are show dogs, I didnt want any problems. The vet talked to a few boxer owners, and we set times to meet "casually " at the vets office. At first we sat on opposite sides of this huge office, correcting growls, calm praise at total ignorance- looking away and laying down. Session two- 30 minutes again we were closer- one correction but good because different dogs. Last visit not one growl. ...