Yes, there is a reason for reason.
The human mind apparently works differently than any other creature. We think in terms of "if this, and if that, then conclusion." This is called a syllogism, and we cannot escape using them because our minds are hard wired to use them--256 of them, no more, no less. You can't think outside of that "box" because there is no "outside". Even if you come up with an absolutely new idea that no one else has ever thought of, you did it using a series of some of those 256 syllogisms.
This usage is what we call "reason". But "rationality" is usually defined as "proper use" of reason, or as "sound reasoning". "Sound" means not just valid thinking, but thinking which can be called "true" according to accepted principles, or previously accepted facts or truths, or of something which appears to be "self-evident".
Only 15 of those 256 syllogisms are "valid". Whether the valid argument you use is "sound" is another matter. I can make a valid case for the existence of unicorns; but it cannot be a "sound" argument because they don't exist.