From what I've seen, there are some that prefer to be the only dog, but for the most part dogs are pack animals. They need companionship. My dogs bark(ed) at other dogs they'd see come to the animal hospital, but bring the dogs to them and they'd be happy.
In another post you mentioned the animal shelter. The place around here has a place for the potential family and pets to be together for a while to see how they get along. I'm not sure, but I wouldn't be surprised it they not only allowed but insisted you bring your other dog to see how they get along.
When we had just Mahana and wanted to get another Doberman, the rescue group had us bring her in to see how she reacted to the others, one at a time.
We tried one for the weekend but he wasn't right for her, so, crying, we took him back. As you know, we did find another (and another!) and she was very happy! She seemed content to be an only dog with some visits from our parents poodle, but she sure loved her new sisters. (Unless they took the prime spot on the couch. . .)
Just my two cents worth...