Cats may also fight even cats that have been best buddies for years. They become more susceptible to illness and disease when under stress. Cats, too, may become unhousetrained, and, in some cases, direct aggression towards their owners, as well as towards the newcomer. Animals are extremely territorial and need enough space to call their own. Problems may arise from, or be aggravated by, forced proximity.
Every pet adds a little more responsibility.
If one has made the decision to add another pet, there are ways to stack the odds in favor of easier acceptance, fewer problems, and more harmonious multi-pet relationships. Some combinations are more likely to succeed than others. The best scenario is to start with two from the beginning. Littermates are ideal, either puppies or kittens. Getting a puppy and a kitten at the same time is good, too, although a very small kitten could be harmed by a rambunctious puppy.