Something I've been thinking about lately is how doing anything well is an unknown factor more difficult than you originally think. Your initial estimate of how hard something is to do is usually terribly wrong. Once you think that you're 20% into a project, you realize that you're only actually 5% into the project. Then later, when you reach what you think is finally 20% again, you realize that, once again, you're only 5% into the project! This repeated discovery of "more to do" really beats people down and discourages them from doing ambitious things.

(See also Hofstadter's law: It always takes longer than you expect, even when you take into account Hofstadter's law.)