“I am not a friend and I am not a servant. I am the Cat who walks by himself and all places are alike to me.”
-Rudyard Kipling, "The Cat That Walked By Himself"
These are some of the most common questions I get whenever a visitor at our zoo encounters an animal that is housed by itself (or appears to be by itself - plenty of times there's another animal in the exhibit that they're missing). Currently at the facility where I work, we have a grand total of five individual mammals or birds (no one ever seems to ask about reptiles) which are housed completely by themselves. Two of those are animals which are awaiting companions that will join them soon; a third is going to be shipped out to join a social group at a different zoo. Still, that leaves two animals which are housed alone. Both are cats. All other animals in our zoo share their enclosure with someone. They might be the only member of their species, but there are other animals with them in a mixed-species exhibit.
Humans are an intensely social species. Solitary confinement is one of the most profound horrors that people can imagine. So it can be hard for us to understand that some animals prefer to be alone. Not as in, they are okay with it, as in, they are not okay with companionship. Many chameleons, for example, can't be housed together for much of the time. In fact, males shouldn't even be housed where they can see each other. At one zoo where I worked, several chameleons were displayed in side-by-side enclosures, but with solid visual partitions between their exhibits. If those partitions were removed, allowing the chameleons to see one another, they instantly became stressed, even though there was still glass between them. Some would rush into the foliage and try to hide; others would gape menacingly and threaten their neighbors.
Just as there are reasons that an animal would be social in the wild, there are also reasons why it pays be solitary. Finding food, for instance - if there isn't much food available, and it's widely scattered, there's no sense in having competition all around you. Protection from predators is another potential benefit. On one hand, if you're in the middle of a big herd, the odds are a predator will take one of your herd-mates instead of you. On the other hand, it's a lot easier for a predator to find a huge herd of animals rather than one lone one.
No wild animal is truly solitary 100 percent of the time. At the very least, males and females come together to court and mate (excluding parthenogenesis). Among mammals, mothers stay with their young for some period of time to raise their offspring. Sometimes animals which are solitary in the wild can live comfortably in social groups in a zoo. Unlike the African great apes - gorillas, chimpanzees, bonobos - orangutans in the wild are highly solitary. The fruiting trees that they feed on are not conducive to group foraging, so it's every ape for themselves. In a zoo, however, with plentiful food, there's no downside to companionship, and social groups can be maintained. The same goes for bears, tigers, and many other solitary animals. Social groups can provide enrichment and stimulation to compensate for other behaviors which a zoo may not be able to allow the expression. A tiger may not be able to hunt blackbuck or chital, but it can tussle and play with another tiger.
Some animals are extremely social and require more socialization than others. Primates and elephants, for example, are very sociable; AZA accreditation standards mandate that elephants be maintained in groups of at least three. A caveat should be offered, however, that all animals - especially highly intelligent ones like elephants and apes - are individuals. There are individual chimpanzees, or elephants, or wolves, or what have you, that simply do not get along with other members of the their species. Maybe there are medical or behavioral reasons also; an older individual, for example, who has outlived companions, by prefer to be housed alone for the remainder of their days rather than integrated into rough and rowdy younger group. In those cases, it's best just to manage that individual animal in a manner that treats it as an individual and takes its happiness and welfare as the primary goal.
As I've mentioned, right now I care for two animals that are more or less permanently solitary at this time. Both have been with other members of their species in the past; both probably will be again at some point. In the meantime, neither seems to be suffering from their solitude. Both belong to species where, in the wild, they might go months without seeing another animal for their kind. Both are cats. They're used to walking by themselves.