Why do we do the things we do?

I am new to this blogging thing, so i'll just write what comes to mind, and what is on my mind is a subject that has been brought up alot between people. Why DO we do the things we do? Psychologist say its from our parents, our surroundings, how we were raised, others say genetics. Why is that when you expect someone to do one thing, they surprise you and do something else? You sometimes think it will be like the movies, where they miracolously know what the other person is thinking and what they will do and save them just in time, but the human mind goes far beyond, and sometimes below our standards. What makes us think the things we think? What gives us a sense of common sense? I have seen cases where a person acts exactly like a parent, they havent even met. Now i am beginning to sound like a stoner, but this is a sign i think too much at times. Sometimes my mind goes into a loop, to a point where my head hurts so bad, i need to stop thinking! The human brain is a wonderous thing, and a mind is a terrible thing to waste. Dont you love how i wavered from the topic?