Your brain conditions itself over time according to the amount of 'trauma' that you're objected to - and it doesn't even have to be your trauma, it could be something on t.v. or something someone tells you. If your brain registers something unusual, it usually goes into an acute shock, so as to deal with the traumatic event and condition itself to deal with that event in the future. This shock is what you felt - the sickness, anger, etc. If you watch that same movie over and over again, eventually you'll find it funny or boring. This means that your brain has conditioned itself to deal with it.