Cats are a bit like people when it comes to friendships. Cats living in a group have “preferred associates” (“friends,” if you will), and other cats from which they actively distance themselves (definitely not preferred). Why would this be, you might ask?
Background probably plays some role. Many species, including cats, learn a lot about their inter- and extra-species relationships during the so-called “sensitive period” of their development. For cats, this is between 2 and 7 weeks of age. Pleasurable exposure to whomever or whatever during this period can lead to lifelong acceptance.
The corollary to this is also true: Unpleasant experiences in early kittenhood can lead to lifelong mistrust or even hatred. Perhaps some of cats' preferences are imbued during the critical period. Extreme examples of how this works are provided by reference to the feral and orphan cat situations. Feral cats that have not been exposed to people during the first 7 weeks of their life will never be entirely comfortable around people. Hand-raised orphan cats that have not had a chance to interact with their own species will likely never be at ease with their kind and instead often become “over-attached' to their human caregivers. Which other felines a cat will tolerate is also likely shaped by early learning, or lack thereof.
This is not to say that social learning cannot occur later, as well. One really bad experience down the road of life can also have profound and long-lasting negative effects on a cat's perception of others, and such fears can generalize.
Other reasons why cats may not get along include dominance, sexuality, and territoriality. As with humans, such factors seem to make the cat's world go round and are so important to some individuals as to border on obsession. A cat that is extremely attached to his owner may not appreciate having to share this valued resource with a new cat, a total stranger. Certainly, a red-blooded male will not appreciate sharing his quarters with another of the same persuasion (thank heavens for neutering). Finally, a despotic leader cat that has his house in order will often not appreciate the addition of another cat, especially if the newcomer isn't fully respectful of the laws that he (or she) has layed down.
When cats are brought together under the same roof they often squabble for a while. This squabbling often takes the form of a few hisses, one cat charging the other, or a few well-directed swats. Spats of aggression are likely before (hopefully) peace breaks out. It has been shown that the frequency of minor spats of aggression tends to decline over 4 months until it reaches baseline. However, peace is not a guaranteed outcome, even with careful engineering of cats' exposure to each other to prevent serious “meltdowns.” Hostilities can and sometimes do escalate until the situation is untenable for the owner or one of the cats. That's when owners call in the veterinary “fire-engine service.”
It's preferable to avoid the development of such entrenched negativity and the following program is designed to do just that:
Once the cats are acting non-chalantly across the screen it is time to progress to the next level - having them in the same room together.
If at any stage of the proceedings there is a negative consequence, then simply return to the previous “safe” level of exposure and hang for a while, days if necessary, until the cats have regained their composure and can be brought closer together once more.
Some people might consider this program to be overkill, but it minimizes all risk of an acute behavioral meltdown and prevents the establishment of any permanent malevolence. Also, if the cats are slated to be friends, it is possible to move through the program more quickly, progressing as fast as the cats' reaction to each other permits. True to the old maxim, it's better to be safe than sorry. Otherwise put: An ounce of caution can save an awful lot of grief - and an awful lot of behaviorist's bills!