People acquire more pets for many reasons. They love them and like having them around. They want to do the best thing for a pet they already have and get another one to keep the first one company while away at work all day. An abandoned animal at a shelter wins their hearts and they know they can't say no to him. And there are advantages to having more than one pet.
Dogs love company, and having a companion can cut down on destructive or annoying behavior.
Cats may play together and amuse themselves while left alone for long periods. But getting a second pet as company for the first does not mean that the animals will require less time from people. All companion animals need to develop and enjoy strong bonds with people, so another animal actually means a larger time commitment from the owner. Someone considering bringing home another pet should also be aware of the potential problems that may develop. The introduction of a new animal is always going to be stressful for a resident pet, and stress has a way of finding an outlet.