Why does this dog dislike being flipped off? I doubt they have a concept of offensive gestures. However, dogs do seem to be sensitive to body language, tone of voice, and overall demeanor. So, while the specific act of "flipping off" a dog wouldn't make it angry, the way you act while doing so might.
The only reason I can think of why this dog gets upset when it is flipped off is that it has had negative experiences with similar actions in the past.
Some dogs don't mind being flipped off. Check out these videos to see the reactions of other dogs who are given the middle finger treatment.
