It varies a lot. Some animals are naturally solitary animals while some are herd animals. Even with fish, many types will be driven mad due to the stress of being alone and will actually die because of it. Many just can NOT be put with another fish (aside from the opposite sex during breeding). You also mentioned rabbits - rabbits (I believe all of them but maybe it varies by type?) are social and need affection daily. People also tend to lump animals like guinea pigs and hamsters together but their needs are so different! Hamsters are solitary by nature, while guinea pigs are very social and need affection from other guinea pigs. Basically, it all comes down to understanding your own pets personal needs and doing research before you buy one. There's no need for someone to buy only one of a certain animal if their research tells them they should be with another. That's just cruel!
