Rats need the company of other rats.
There are no two ways about this, and one should never buy a rat with the intention of keeping it alone.
Rats are very social animals, and must live in same sex pairs or groups. In the wild, they live in large family groups. They sleep together, groom one another, play with each other and form strong bonds. Even many laboratories will now refuse to keep rats alone.

However, despite this, many people still go out and buy a single rat, and some websites, pet shops and books still perpetuate the myth that this is ok.
I've heard many reasons as to why people only want a single rat. The most common is that people seem to think that by just having one rat, it will bond more closely to them. This is simply untrue, as anyone who has more than one rat will testify.
Besides, don't you want a rat to bond to you because it truly loves being with you, not just because it has no one else to be friends with?

Some people seem to think they can overcome the issue of loneliness by simply spending more time with the animal.
But even the most dedicated owner simply cannot spend all day every day with their rat. One of the most active times for a rat is at night, when his owner will be asleep.
What is he to do alone for 8 hours or more? How about when his owner is at work during the day? Even if a person were able to spend all day with their rat, the animal would still miss out on things that only another rat can provide.
Rats love their people, but at the same time, we are a different species. It would be like expecting a human to live with only chimpanzees for company! There would be a certain level of communication and bonding, but nothing would compare with having another human to interact with. Rats kept alone can often develop problems such as depression, and even self mutilation like pulling out their own fur. There is evidence to suggest lone rats live shorter lives and have more health problems than rats housed with others.
Being alone is completely unnatural to a rat, so it is hardly surprising that such a lifestyle would cause problems.

Some people think that having more than one rat is more trouble and more work. This is also not true. Two rats are no more trouble than one. Any cage that is large enough for one rat is usually large enough for two, and a pair of rats only need cleaning once a week, which is no more often than a lone rat. In fact, two or more rats are far more entertaining as pets when you can watch them interact and play with one another.

As small rodents go, rats are pretty amiable. Two males can live together perfectly happily, as can two females. Adults can happily live with babies, and neutered males will happily live with females.
If introduced as babies, rats will usually accept other rats without question. Introducing adults to one another can sometimes be a little more tricky, but certainly not impossible. See the page on intros for more information on that.

There are occasional very rare cases in which rats will flat out refuse to live with other rats.
These are almost always old males who are set in their ways, perhaps old rescue boys who have spent their whole life alone and find it hard to adjust to having to share with another. However, most rats like this can be successfully integrated with other rats with enough time and patience. There are very occasional exceptions to this, or perhaps situations in which the rat is so old that the stress of introducing it to new rats might make it more unwell. In 12 years of rat keeping, I've only ever had one boy who would not tolerate other rats, and even then, I did not push him too far as he was very old and I didn't want him to spend his last days stressed out with intros.
In the situations where a rat will not tolerate any other rats, all we can do as owners is to spend as much time with the animal as we possibly can. Fortunately, these situations are the exception, not the rule.
Neutering a buck can often solve the problem of rat aggression, and many people believe the risk associated with surgery is worth taking in order that the rat not have to spend its life alone.

If you do nothing else for your rat, get it a companion. Company is second only to food and water in the list of a rat's requirements. This is not an opinion, it is a fact.
If you are not prepared to get at least two rats, do not get rats at all and perhaps get a hamster instead!