Do you think most salaries are based more on the difficulty of the job or the value of the service or product?

I guess to clarify, do you believe its how hard you work that counts? Or do you believe it has more to do with what your work is. For instance a Janitor could work very hard his entire life and still fail to make the kind of money some ceo's make in a month. Now obviously one often has to work hard to become a CEO but does the work really reflect the level of pay? Personally I would argue that it generally doesn't. But I'm curious what you guys think.