Behavior is determined by the current stimulus situation and past reinforcement history.
The market is a pendulum that forever swings between unsustainable optimism (which makes stocks too expensive) and unjustified pessimism (which makes them too cheap). The intelligent investor is a realist who sells to optimists and buys from pessimists.
Never risk more than you can afford to lose.
"We are all broken, that's how the light gets in."