We've always had two (or more) dogs, and prefer it for many of the reasons mentioned by others here.
One tough issue when having two, though, is when one dies. Depending on the remaining dog, there might be a LOT of anxiety and mourning that occurs. We just went through this, after losing our older Ridgeback in May of 2011 to lymphoma. It took our remaining dog perhaps a year to stabilize, with drugs and lots of work. We'd been on the list for a new puppy since June of 2011, and finally brought Zinny home in August of this year--Koa has been much better since then and we have been able to stop her medication. If I'd known how long it was going to be before the new puppy was available, I would have gone the foster dog route.
IOW, once you go to two dogs, it might be tough to go back to one, depending on the temperament of your dog.
Simkie makes a good point. My older guy was an only dog for most of his life and could pretty much take or leave having another dog around. But our younger dog has always been part of a 2 dog house and he derives a lot of comfort and "braveness" if you will from the older dog. I think that when our older guy goes, we are going to have some major issues with our SP being depressed, anxious, and not eating. He doesn't handle stress well now, but mostly takes his lead from the lab.
A good horseman doesn't have to tell anyone...the horse already knows.
My poodle went from being an only dog to living in a house with three other dogs, two poodles and a lab. One is his size (15lbs) and they get along fantastically. They wrestle and tug and act like goofballs non stop. They absolutely love each other. If one is taken somewhere, the first being they greet upon returning is the other one. The two big dogs are kind of indifferent. The standard poodle and the other mini seem to like each other, but the standard is 6yo and is kind of beyond the constant playing. The lab is 13 and likes to play with the humans, but is uninterested in playing with the other dogs. The two minis are pretty inseparable though.
The female mini is kind of a terrorist and is the undisputed ruler of all the things. She's completely fearless and will march up to the other dogs and bully them off food, take toys, steal treats, etc. if the humans aren't actively stopping it. She's not particularly aggressive about it, just walks up, grabs the thing and walks off. The others just kind of dumbly look at her. The other three are all content to leave everybody else alone with their toys/treats/food/whatever. The intact male mini is *starting* to challenge her a little when she tries to take things from him, but his response is to lie down on whatever it is so she just can't get it. Even he backs off though if she is pushy enough.
The two female poodles (standard and mini) are crated together and the male is by himself. The sorta senile lab is left to roam the house/sleep on whatever piece of furniture she deems best. Nobody ever fights or really argues about anything.
I have five dogs; have never had less than two... I can't imagine having only one... Mine have such a dynamic little society among themselves, they sleep together, draw confidence and comfort from each other, learn from each other, steal each others toys... My bosses have one dog and they bring him to work and leave him in an office or an outside kennel and I always feel so sorry for him, even though he can see and hear them most of the day he's lonely and scared a lot (rat terrier)... My dogs always have each other when they don't have me.... they're all pretty tight despite the usual bumps in the road, they're a family and they all love puppies-any newbie gets loved on and played with and they're happy to see someone new.
Multiple dogs work is they are a balanced pack of dogs with a strong leader in their human. I usually have 3 dogs. I occasionally take in a foster. I've had dogs who could care less about the rest. Others are quite social with each other. I have a friend whose pack ranges from 6-10 dogs depending on fosters. She is a very good leader and she has no trouble moving dogs in and out. On the other hand people who treat their dogs like " live stuffed animals" can have problems. I wouldn't get a dog for another one. Mine all have individual relationships with me. When they first become part of the family I spend a great deal of one on one time with them. As time goes on I don't do this as much. I still work with them individually. They know I "own" them not the other way around. This is what has worked for me for many years and dogs.