Cats are not pack animals like dogs, but they are very social animals. They don’t distinguish between species, so they can be social stimulated by another cat or a human or a dog. A lot of it also depends on what the cat is use to, if you leave a cat alone that is use to you being around all the time, it will get lonely. Cats feel loss if the cat / dog / human they live with dies, sometimes enough that they will stop eating and starve themself to death.