The fundamental aim of Kant's ethical theory is to determine how a command can be a moral command with a particularly obligating character. But morality should be the kind of thing which does not vary from individual to individual, because otherwise there would be no point in providing a reason for behavior other than that it is simply what one wants to do. In other words, it would be to acknowledge that there is no reason for acting one way rather than another. But because we are rational beings, we can give reasons for what we do, and we can act based on those reasons rather than acting simply because we want to. Being rational means being able to act based on motives that are universally defensible.
Morality presumes the existence of rational persons.
A society of autonomous, rational persons is a kingdom of ends, a society in which each individual has dignity, intrinsic worth, and is considered fully responsible for his or her choices in the making of universal law. The possibility for rationality makes humans morally significant, but that does not indicate specific morally justifiable ways to act. We are free to choose our moral principles. If we choose unreason, then there will be no point in asking for reasons for why we act. This is called the naturalistic fallacy because it takes something that is part of nature and tries to make a claim that transcends nature.