If you really pay attention to the process of thinking, what you'll see is that thoughts just happen, whether we want them to or not, we can't stop them and we have no idea what they're going to be until they show up in consciousness. I mentioned previously, but how often are you doing something (work, watching TV, reading, etc) and some random thought suddenly pops into consciousness? "I really should stop drinking so much soda" or "Did I close the garage door when I got home" or "I should mow the front yard soon. It's looking pretty bad".
What we call reasoning is just the flow of subsequent thoughts, after the initial thought, arising in consciousness, which we also don't control or initiate. They just happen as the initial thought did and those thoughts are dependent on the entirety of every external influence that has structured the neurological structure of your brain.
In order to have free will, there would have to be a self that exists outside the flow of thought/consciousness and had some visibility/control over the neurological activity that creates thoughts. There is no evidence of an I or self and what we subjectively view I and self is just our experience of thoughts flowing through consciousness.
As an experiment on the existence of free will, if you're willing to play along, take a few seconds, minute, whatever (before reading below) to think of some movies.
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So, the first thing to be aware of is that this is about as free a choice as you'll ever make. The only real limitation is that you can't select movies that you've never heard of or don't exist, but you are quite literally free to pick ANY movies you want. No outside force is restricting you in any way.
Now, assuming you played along and have some movie titles in mind, here's the question: Were you free to pick a movie that didn't occur to you to pick? In other words, as movie titles were entering your consciousness, could you have picked one that didn't occur to you? For example, you've most definitely heard of Wizard of Oz, but due to the current status of your neurons in your brain, Wizard of Oz didn't occur to you.