Some male rats (like mine) can be very territorial if they're not fixed, and he had a problem in the past with another male wanting to terrorize him, thus I keep him alone but I also take him for outings a few times a week to parks and fields and all sorts of adventures. I also meet with other rat owners for play dates and such... Sometimes I feel bad for keeping him to himself, but he gets very antsy around others and I am constantly checking him for any psychological disorders that I can... He hasn't acted depressed or anything though, and he really loves interacting with the mice (he's a gentle giant)