I don't know about all dogs, but our second dog was the best thing we ever did for our first dog! Our older dog (Max) was abused before we got him. We'd had him for two years and he was a total sweetheart (still is, I've never had a problem with him) but he was still *very* skittish. He'd hide a lot of the time and even though he loved being petted, he was scared to come out to be petted!
Once we got our second dog (Maddie) (who is a PITA, but she's cute so we love her anyway
) Max totally came out of his shell. They play together all the time, which cracks me up because Max is 120 lbs and Maddie is 15 lbs. But overall, Max seems soooo much happier. He comes out more and hangs out with the family, he'll set his head on the arm of the couch to be petted, stuff like that. He doesn't seem scared all the time anymore. And honestly, they distract each other and play together so they don't need as much attention from me. Maddie is always a total people dog, but if Max isn't around she is much more needy.
Anyway, we have more shedding and our dog food bills have gone up, and Maddie sometimes chews things and jumps on people, but overall it isn't much more difficult than one dog. I am not sure I'm a dog person and I don't know if I'll have dogs after these two are gone, but if I do I'll get a pair of them again.

Once we got our second dog (Maddie) (who is a PITA, but she's cute so we love her anyway

Anyway, we have more shedding and our dog food bills have gone up, and Maddie sometimes chews things and jumps on people, but overall it isn't much more difficult than one dog. I am not sure I'm a dog person and I don't know if I'll have dogs after these two are gone, but if I do I'll get a pair of them again.