This is beginning to sound a bit like the "chicken or the egg". I personally don't subscribe to the idea that a "chemical reaction" causes me to think in a certain way. I think it's the other way around. A thought produces a feeling (via chemical reaction), and not vice versa.
Thought is a continuum, so to ask that question at any point except the beginning won't give you an answer. Given neurophys studies, I think it is quite safe to say that without the chemistry you wouldn't have the thought. Whether you measure charge across a neuron and see its mechanical and electrical effect or you look do an MRI while doing some activity, you can see the chemical changes. If thought isn't tied to the chemistry, what do you think it is physically?
When it comes to addiction of things like cocaine, etc. we know it isn't thoughts that cause it. Many studies have measured notable changes in molecules associated with such addictions (it is why you see higher addiction rates with something like cocaine than say alcohol- if it was just thoughts wouldn't you expect the same addiction rate for abused substances?). So then the question is what addictions are you saying that thoughts cause?This sequence of events (or chemical reactions) that facilitated that thought are irrelevant to the discussion of addiction (in my opinion).