It is also believed that pet rats fare better emotionally when kept in groups, which not only keeps them happy, but healthy, too! Single rats often seem to be more easily stressed and can be more prone to disease. Some rats can even develop behavioral problems or neurotic tendencies when kept alone. Since caring for two rats is no more difficult than caring for one, and the extra cost is nominal compared to the benefits of having two together, there really is no reason not to get two or more. As long as you give them attention individually, they will each bond with you. In fact, many people believe that keeping two rats actually helps them bond to you more easily.
Though it is very possible to introduce a new rat as a companion for a single rat, it is much easier to get both at the same time, preferably from the same litter as they have grown up together and already know each other!
It's Vitally Important!
An only rat is a lonely rat! Rats should always be kept as pets in same-sex groups of two or more, for many reasons. In the wild, rats live in immense colonies, and naturally need and enjoy the company of other rats. They groom each other, sleep together and play together. No amount of human attention can replace the company of another rat. Therefore, pet rats are better off with at least one other for companionship.