Jackson was not an "only" dog until he was 10 months old when my moms dog (living with us) died. We would split our time between my dads and moms house and both had other dogs. But our "main" house is at moms. After she died, he was really lonely for a while. He always seemed so bored and sad. But I think he adjusted well. However, I do get him to the dog park once a week or more, he also sees the dogs that I dogsit almost every day, plus I take him to my dads house two nights a week where he gets to spend the day/night w/ Lilly and Buddy. So basically I tell myself not to feel too guilty -- he lives an amazing life and is spoiled rotten!
But I do want to get another soon when I begin working more so he will have a permanent buddy.