They seem to cherish every second spent playing with their humans or rat companions, exploring or causing trouble. One of the easiest ways to give a rat more opportunities to enjoy life is to make sure it has another rat to live with. It saddens me a great deal that many rats live alone simply because their owners do not know that they are far happier with company. Rats are highly social animals which gain a great deal of enjoyment and stimulation from each other's company. It just isn't natural for them to live alone.
Rats play, groom each other, and cuddle up to sleep together, keeping each other warm.
They also communicate by touch and smell, and by sound at frequencies we can't hear. No amount of human attention can entirely replace the company of another rat. Rats with company live more full and active lives than those kept alone. There are many experiences open to them which the single rat simply cannot have. And no matter how much attention a person gives a single rat, she will not be available 24 hours a day and willing to play in the middle of the night when rats are at their most active.