This is the title of a new book by Harvard psychologist Daniel Gilbert. One quote from the N.Y. Times review by Scott Stossel in the May 7 book review section: "When we have an experience..on successive occasions, we quickly begin to adapt to it, and the experience yields less pleasure each time...Psychologists call this habituation, economists call it declining marginal utility, and the rest of us call it marriage."
His basic theme is that humans are very bad at predicting what will make them happy. Things we expect to give us joy make us less happy than we think; and things that we dread make us less unhappy, especially after some time has passed. There is a "psychological immune system" that starts up after big negative events.
From the book: "How do we manage to think of ourselves as great drivers, talented lovers and brilliant chefs when the facts of our lives include a pathetic parade of dented cars, disappointed partners and deflated souffles?...The answer is simple: We cook the facts." What gets us through life is just the right amount of delusion, enough to fool us into feeling relatively good about ourselves. Interestingly, the clinically depressed seem less susceptible to these basic cognitive errors.