Evolution has hidden post-hypnotic suggestions in your behavior.

You may think you're the absolute master of your emotions, but that whole "consciousness" thing is just a thin scraping of self-awareness over a huge network of evolved drives and compulsions. 

Some excellent experiments in behavioral research were conducted by Professor Susan Mineka in the eighties.

By showing some monkeys footage of a wild monkey utterly terrified of snakes, she triggered the same hysterical responses in those who had never seen the object of fear,

Further, attempts to trigger a fear of flowers by showing fake footage of a monkey scared of plants failed. It seemed that the "snakes suck" wiring was always there, but until it was externally triggered it never manifested.

The same research also showed how to combat these phobic trip-switches:  by showing them a monkey that wasn't scared of snakes, even if that was a fake monkey, the terror-reaction was strongly reduced.