If they get along, they can play with one another (dogs like to do this). If your first dog is lonesome, then he will not be as lonesome if you have a second dog (but sometimes they are not as thrilled with the idea as you might anticipate).
If you love dogs, then having a second dog will add to that fun.
Different personalities of dogs may suit the different people in the household.
More Doggie Love and kisses for you.
More walking and maintenance
More vet bills and trips to the vet
Make sure you know what you will do when you go on vacation or out of town. Not all dogs like to go to "THE KENNEL", and many kennels are booked on holidays, unless you plan in advance.
More training, obedience, etc.
You might consider fostering a dog or two for your local Animal Rescue League to see what having an additional set of paws is like in your home. This would give you a little preparation without the requirement of keeping the animal.