If you truly understand dogs and cats, you understand their behavior when confronted with new sights and sounds which are foreign to them.
By putting the word aggression in front of an animal, you sentence it to death in most cases. The problem with the word is the frequency in which it is used by inexperienced "animal lovers" who think they are doing the most humane thing for the dog.
I remember showing a father and his two daughters around the previous shelter I was at and he kept saying over and over, "I don't want an aggressive dog. I can't have an aggressive dog. Don't show me any aggressive dogs." This continued throughout the whole walk we took until finally I asked him what he meant by aggressive. He said, "I don't want a dog who jumps!"
Aggression? Jumping? Not quite the same thing,
When you have newbies in the animal rescue field, who have had little to no exposure to animals who are strays, or have been dumped off due to stupid owners, or their owners have died, you have people who have never seen cage aggression, fear, sadness or anything else of that nature. They may have always had pets and loved them, but were never around shelter dogs and cats.
This does not bode well for the animals now in their care.
Any dog or cat, given certain situations, will bite or scratch. When it happens in a shelter, they normally die.
Behavior modification is one of the main programs which should be installed in shelters today. All animals benefit by using enrichment programs, regardless of their backgrounds. There are a myriad of reasons animals are euthanized, but behavior is the number one. It seems like in this progressive era of animal care, we could change that.