I think a lot of people don't do as well as they could, just because they're satisfied with a set of grades that is less than what they're capable of, at their full potential.
Surely the best attitude to have is to think that straight A*s are acceptable/to be expected, and anything less than that is a bit of a slip-up. If you think that a B is a perfectly acceptable, or good grade, for example, you're not as likely to be motivated or have the incentive to get anything higher than that. Why lower your sights?