Choose to be nice?

I had a patient the other day (week, month.. I dunno.. time runs together) and when I received her, the facility I picked her up from told me she used to be the nicest person, but since she got sick, that she had become mean. And she was. Everyone was blaming her condition on her change. So I got thinking, if someone was a complete asshole before falling ill, then became the nicest person, it`s often attributed to an awakening of sorts. That they suddenly become aware of they way they were, and now want to try to right wrongs and live a good life from then on. If someone who was always pleasant, then falls ill and becomes miserable, it`s sometimes attributed to the disease.

I got thinking, maybe the meanie who became nice did not choose to, but is actually suffering from the disease process, and the niceness is a side effect and not a conscious change of personality. I have absolutely nothing to back this theory up, but I found it interesting to think about. Do we chose how we are, or is it ingrained in us and we really have no choice in whether we are nice or not?