The laws of nature are mathematical in nature. They do not allow for multiple options, they just have one correct solution for any given event. One can argue that this is not the case in quantum mechanics, which describes systems in terms of probability rather than just one specific answer, but even then, those probabilities are themselves mathematically determined and there can only be one correct answer for the description of a system, it's just that the one correct answer is in the form of a wave function rather than a particle with a specific location.
Events as they are perceived by human beings (such as what I am doing right now, typing a comment) involve ridiculously large numbers of sub-atomic particles, each of which can be described only by means of a very complicated mathematical calculation. So while events may be deterministic in theory, that does not mean that it is necessarily practical for us to do the calculations to determine what is going to happen. However in theory, if we had enough information and enough computational capacity, we could calculate the outcome of literally anything.
Instead of asking someone what they want to eat for lunch, if we knew the state of every particle in that person's brain, we could calculate the answer. Particles can only do what the laws of nature require them to do, and all things are made of those particles. This situation is sometimes described as a "clockwork universe". The universe may be like a huge machine, that operates in an inescapable, pre-ordained manner, however unpredictable and chaotic is may appear to human observers.
It is possible in theory that every single event, large or small, which has happened in the past 13.8 billion years since the Big Bang, and every event that ever will happen in the future, happens necessarily by reason of the laws of nature. And if that is the case, then human beings do not have free will. Although we obviously do make decisions, we always make the decisions that we must inevitably make due to the various biochemical processes taking place in the brain, which are themselves the product of even more basic interactions of sub-atomic particles.
The human mind and personality could be simply a part of the clockwork universe. We may just inescapably do what the laws of nature cause us to do. Thus, we have no true independence of thought, only an illusion of independence of thought. That is the theory.
I do not know that this theory is true. There may be something else going on which we have not discovered or which we do not fully understand as yet. And maybe quantum uncertainly is more of a problem for the deterministic universe than I have described it to be. But maybe it is true that there is no such thing as free will. Even if that is true, in practical terms we still have to live our lives as if we did have free will.