Our chocolate lab Chase does not like boxers.  He’s very sweet with all people and as far as I know he likes all other breeds, just not boxers.  About five years ago he met a boxer he didn’t like and he hasn’t liked any boxer since.  He attacks them on sight with no provocation.  He recently attacked one at the kennel and the other dog needed stitches.  After that the kennel worked hard to keep him seperate from the other two boxers boarding there but he would still bark and try to lunge at them through the bars.  My guess is that the first boxer displayed aggressive body posture and he’s carried it over to other boxers.  Usually if dogs get in a fight with another dog, they carry that aggression over to most other dogs.  Ours seems to have limited the aggression to one breed.  Here are some articles about dog aggression and some ways you can correct it:

• My dog is dog aggressive – What Can I do about this aggression?.
• Aggression: Social Aggression to Unfamiliar Dogs
• Controlling aggressive dogs