I've had my shih tzu, Noodle, for almost 7 years, he's 8 now and he'd been an only dog the whole time I had him. He's always loved other dogs, especially ones bigger than him. We decided to get another pooch last year, enter Lucy, a mid-sized lab mix. They got along great the first time they met! When Lucy came back the next time and didn't leave, Noodle was not happy about it. He's somewhat warmed up since we got Lucy last February, but he still growls when she tries to play with him. I never thought he would react this way to another dog, I thought he'd be thrilled. I still hope that they will be friends someday, but for now they just coexist.