Goldfish are very social creatures. They really do enjoy having a buddy. I would actually do a trio, and I just found this out first hand a few days ago. I had to QT one fish to treat a bad finrot infection, and since I only have a pair that meant both fish were alone. These two guys have lived together for going on 4 years now and have never been separated. By themselves both fish were much more skittish, and I even had trouble getting one to eat (this never happens with goldfish). If I had a trio this wouldn't have been so bad on the fish left in the tank. The other reason to have more than 2 is to spread out any aggression. Goldfish can get kinda nippy with each other around mealtime.