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.
My mother was an atheist. She used to say that there was good news and bad news about hell. The good news is, hell is just the product of a morbid human imagination. The bad news is, whatever humans can imagine, they can usually create.