A lot of people get one cat and they get used to being alone. People who get more than one cat generally get them from the same litter.

But cats are social animals and will absolutely bond with each other. Even cats not from the same litter will bond with other cats. Usually it takes a careful and patient introduction, which people don't realize. All they'll do is put the two cats in the same room, observe them fight, and come to the conclusion that their cat just wants to be alone. But that probably isn't the case at all.

Like many other animal (including human) behaviors, cats bond the easiest when young. Littermates, obviously, but unrelated cats as well. Second easiest is to bond an adult cat with a kitten. Introducing two adult cats to each other? It can take time and skill (doing it gradually is the key!), and depending on their individual personalities, they may become best friends, or only scarcely learn to tolerate each other.