Our capacity for reason is a result of our rational souls, the souls which God gave us.
You would not say that an animal is capable of true reason. They are capable of following instinct, and do not make distinctions between right and wrong. As Catholics, we also believe that they are lacking a rational soul, one which is capable of knowing God. (Incidentally, this is also a defense against people who say that homosexuality happens in nature so it can’t be wrong; animals are not bound by the moral laws that we, as people with rational souls capable of knowing right and wrong, are bound by, so saying it happens in nature is equivalent to saying that foxes have red coats; a worthless observation)
Our rational souls imbued us with free wills and the capacity for choice. With the ability to chose came the need to distinguish between right and wrong. As a result, we were given the ability to reason so that we may reach the proper conclusions about the morality of our actions. Despite what anyone may tell you in our relativistic age, if a person truly sits down and applies reason to a problem, void of their predefined prejudices and notions, everyone would reach the same conclusion about issues of morality. This is a result of the objective truths the exists in the universe that which we are all called to live by.
Hope this helps.