My previous dog, Jake, was much happier as an only dog. He was much more playful before Harley came into the picture, and once he had to compete for toys, he just stopped playing altogether. After Harley died, it was obvious that he was content alone.
This time around, I am going to try to be more in tune with how my dog might react. Based on Theo's behavior now, I wouldn't bring another dog into the picture. He likes to play in group, but he enjoys all the attention at home now. Maybe that will change as he matures, maybe not.