Your brain is composed of just about a trillion cells, that’s 1,000,000,000,000 cells. Only about one hundred billion, that’s 100,000,000,000 are neurons. The other nine hundred billion are a support system for the neurons. Without going into cellular architecture, it is good to know that each neuron can have up to 20,000 dendrite branches, or 20,000 potential connections with other neurons which are critical to storing knowledge.
One would think that all of this machinery performs well in all situations, and is relatively accurate and reliable, however your brain frequently presents you with visual disinformation as well as actively altering your conscious decision making facilities.

When our brain is presented with stimuli that it interprets as a significant threat it triggers what is called a fight or flight reaction. Our bodies prepare for a hasty retreat (flight) or a round or two of hand to hand (fight.) While there is several bodily change that happen during a fight or flight reaction (blood pressure jumps, you begin to sweat, and your pupils dilate) the most interesting ones happen in the brain. Endorphins are released into the blood stream, these natural pain killers help us withstand pain in the event that we need to fight, and our higher judgment system is turned essentially off. That’s right, your brain turns off the inner philosopher and our most primitive responses take over. This is not a time for deep thought, it is a time for action, and our brain has prepared us to act. Experts say that most human beings experience a fight or flight reaction just about 6 times a year, however this is entirely dependent on your career and stress levels.

A close relation to the fight or flight reaction is called the amygdala bypass. If someone has ever told you they don’t know what happened, they just acted, this is probably the cause. When sensory data enters the brain, it passes through the thalamus (except the sense of smell for some reason) which routs the information to the amygdala and the area of the cortex (the area that governs all your sensory input) that is relevant to the input. The amygdala can, if it believes there is a threat, shut down the area of the cortex assigned to analyzing the stimulus, and force you to act. Most people don’t remember why they acted, or wanting to, they just remember acting without thinking. While these actions take place only in times of crisis, your brain continues to influence your conscious behavior every day.

Our brain has a nifty little device called the urge mechanism. The urge mechanism starts when your body sees something desirable or when your blood sugar drops. Your thalymus which is part of your limbic system or leopard brain converts the physical need into an urge within your cortex, this is your brain whispering “I WANT SUGAR!” into your conscious thought. These urges are quite powerful and push other matters out of the way, leaving you with a feeling of emptiness, a gnawing feeling in your gut. Next, the cortex translates these needs into a targeted desire for something specific. Once this urge grows powerful enough, you take action, your brain wants the action to continue, so you keep eating, feeling satisfaction every time you complete the action, or take another bite. Finally, when you have all that you need, your brain tells you to stop, and a feeling of fulfillment or that of being “full” takes over. This is when you look down at your hands, and realize you just ate an entire pint of ice cream. The brain plays its little tricks on us all the time, influencing what you do, and what goes on around you. Like any good node, I will close by talking about sex.

For all the males of the population out there: As I hope you know, an erection is maintained by pumping blood into the penis. But where does this blood come from? If you guessed your brain, you are right! When a man is erect, the part of his brain that controls his higher functions is impaired, most specifically, the part that would keep him from mating. Your brain cripples itself, and impairs your judgment when you become aroused simply to get you to “go for it” and spread your genes.

But don't worry, because you can fight back. Put a pen across your mouth in your teeth with your cheeks bunched up comfortably behind it. If you hold it there for a short while, you will actually make yourself "happy." This is because your brain is conditioned to feeling happy while you are smiling. If you fake a smile by holding your cheeks back with a pen, your brain falls into the trap, and assumes you are happy. Your brain is the product of hundreds of generations and thousands of years of evolution. While we may not like everything that it does to us, it can be explained, and is generally in our best intrest. The brain is an amazing piece of flesh and you should use it well!

Sources:
http://changingminds.org/explanations/brain/brain.htm