Most people consider cats to be asocial or even antisocial.
Neither of these assumptions is strictly true. Cats should also not be compared with dogs who live their lives as true pack animals. Although cats often form quite large, peaceful and very stable social groups. However, the social structure takes time to become stable, and once stabilized, the cats like it to remain that way. This may be why the only newcomers which are readily tolerated into an already existing group of cats are kittens that are born into and grew up within that social group.