Used pets have already been taught two or more ways of reacting to any given situation, and they come with baggage. They have fears or hurts that we do not know about, so when we do something, this animal's response may not make sense to us. You reach out to pet the dog and he wets all over your shoes. But perhaps this was the beginning of someone beating the dog at a previous time in it's life. Treatment for behavioral problems needs to be designed on an individual basis.
Animals arrive at their behavior from different circumstances.
Correcting all snapping dogs or scratching cats in the same way simply won't work. Each animal has an individual temperament and each owner of these animals have different attitudes and behaviors. Time is needed to work out these problems. Some animals need a firm hand during correction, others are so threatened by simple comands that this simply will not work. In order to get the problems solved, communication is essential.