When people create personal expectations for themselves.
For some, a C is what they want, other students really want an A*. For both cases, anything below that grade will be disappointing, but it's just harder to accept below a C because colleges and sixth forms will consider it a 'fail' which isn't nice to hear at all. Whereas, the student who wanted an A* and perhaps got a B would be a bit disappointed, but the B is still a very good pass.
Basically, it all depends on what you're aiming for.