I can make a decision that will ruin my life. I have that choice. I can do it knowingly too, even if I know that no good outcomes will come as a result of the decision. Most likely I won't do it, but who's to say I won't?

The brain is not fully understood yet, and no one is really sure on how it works exactly. Until we get to the point where we can understand the human brain so much, that it can be built in a lab resulting in a living person(along with a body, or just give some idiot a new brain :)), I don't think we can explain how it works in this forum.