Most helpful to me was the part about how people tend to project how they’re feeling right now into calculating how happy they’ll be doing something in the future—leading to obvious miscalculations. For ex:

“Indeed, one of the hallmarks of depression is that whhen depressed people think about future events, they cannot imagine liking them very much…. because when she imagines the future, she finds it difficult to feel happy today and thus difficult to believe that she will feel happy tomorrow.”