You don't need a man, just as a man doesn't need a woman- most people just like to have someone else to have around and have a close relationship with (usually more intimate than that with a dog- dogs can't talk back after all so conversation can be pretty one-sided!) If all your needs, social, emotional etc. are being fulfilled, then there's no need for a partner- but most people don't have that kind of fulfillment from everyday life.