Rats have an instinctual group mentality. They are social animals by nature, and it is very unnatural for them to live solitary lives. It would be synonymous with a human living in solitary confinement. They are more intelligent than your normal "pocket pet", and require stimulation, entertainment and interaction that they can only find in another rat(s). Yes, they can bond closely with their humans. But we can never replace their need for rat-to-rat interaction. They help to groom one another (especially in spots they can't reach, like between the shoulders), and provide a sense of security and well being for one another.
A lone rat will also likely become very bored. Boredom can possibly lead to bad habits (cage bar chewing, barbering) and possibly depression. Can you imagine finding yourself being kept alone as the only member of your species? Toys and interaction with owners are wonderful, but the need to communicate and bond with members of your own species are crucial. And yes, rats do communicate with each other! If you were ever to use a bat detector to "eavesdrop" on them, you'd hear them chattering and squeaking "conversations" all day on frequencies we cannot hear.
This classic vid sums it up without too many words. The bonds they develop with one another are plain as day.
[ame]https://www.youtube.com/watch?v=cDy3bMRuobE[/ame]
