That humans are "rational beings" does not mean that people always behave rationally. It merely means that, fundamentally, reason, the rational faculty, is the human tool of survival.
For example, while animals have instincts, and plants have photosynthesis, humans have no comparable form of "automatic" survival. Men and women must think (and then act) in order to remain alive. But unlike animals, that cannot set aside their instincts, or plants, that cannot renounce photosynthesis, humans can choose to be irrational or renounce reason at any moment, at any step. So, paradoxically, man's ability to reason--to think--also includes the ability to evade, or not think. This is the real meaning of what some call "free will."