Rational beings are ends in themselves

In one of Kant’s books he talks about two ways of expressing the categorical imperative. Including not using people as tools, Kant is saying that in a moral world all people have intrinsic value. If you are using someone to achieve a goal you are giving them instrumental value. They are only a means to an end. This is another way of expressing the categorical imperative because one, you are universalizing your maxim; and if you are refusing to treat others merely as means to an end, you are also universalizing a maxim and a very fundamental one. Second, both maxims may be interpreted as expressions of the Golden Rule.
Humans are considered value-givers and this makes them rational beings. All rational beings have absolute value. The second formulation of the categorical imperative is also respecting yourself and your own values because as a rational being you have the right to set your own values. This means that although making sure that you are not using people as tools, you need to make sure that people aren’t using you as a tool.
Nonhuman animals don’t belong in the moral universe at all; they are classified as things and can be used as a tool by a rational person because animals cant place value on something.
The third major theme of Kant’s work is the “kingdom of ends” applying that the categorical imperative is something all rational beings can do, then all will end up following the same good rules because all have universalized intentions.
I think a lot of what Kant mentions makes a lot of sense. People should be respected for who they are and not what they can do for you. However, as psychological egoists this would be hard because we are inclined to use people as instruments in some situations if it is in our self-interest.