We do have reason, in as much as our brains assess each situation based on all prior experiences, and our thoughts are definitely rewritten, or at least impacted, by every new experience. However, we have no visibility to how or how much any experience impacts our future thoughts and, therefore, our actions.
To say "there is definitely free will" would mean that there is an "I" that exists somewhere behind our eyes (at least that's how most people imagine it). And that "I" has the ability to think our thoughts before they enter consciousness, which is when "we" think them. In reality, there is no "I" that exists outside of the stream of thought. There is only the stream of thought. It's happening constantly, we can't stop the stream of thought and we have no idea what the next thought is going to be until our brain pushes it into consciousness.
How many times have you been sitting on the couch, watching TV, and you suddenly have a random thought like "Did I close the garage door when I got home?" There is no "I" that consciously created that thought or had an awareness of that thought before it existed in consciousness, yet that thought likely resulted in an action - you getting up and checking the garage door. Or, that thought resulted in several subsequent thoughts, possibly thoughts reaching back into memory to determine if you DID close the garage door, but you didn't control, and couldn't prevent, any of those thoughts either.