Why there must be free will The sense of the word "do" is what as at question here.
I am asserting that there is such a thing as subjective causation. I'm sure that it does 'mesh' with the deterministic schema within which your evaluations take place, but in some supervenient context. ie. Just as the deterministic processes of chemical interactions are not inferable from the laws of quantum mechanics.
The laws of quantum mechanics play out, eventually systems emerge (at a different scope and scale) in which different sets of laws dominate (baryonic matter). Again more complex systems emerge with new laws not entailed by those of the 'parent system'. Etc. Eventually you get to chemical systems, biological systems, life, consciousness. Ultimately I believe these systems can be reconciled under a broad enough theoretical framework. Intertheoretic Reduction should be possible. The process I described is, in broad strokes, an accurate depiction of how the universe has evolved.
For me, all facts are provisional, in that our knowledge is only ever approximate (a priori excluded, I'm more interested in 'experimentalism' as a philosophy of existence). One thing that history shows us is that we never know as much as we think we do. Once our context of understanding expands sufficiently some facts may assume a whole new meaning. Paradigm shift.