Quirrell:

This truly dangerous wizard shall perhaps be bent on some project of which he anticipates great renown, and the certain prospect of losing that renown and living out his life in obscurity will seem to him more vivid, more aversive, than the unknown prospect of destroying his country.

The Sorting Hat:

“You know—you aren’t letting yourself think it, but in some quiet corner of your mind you know just exactly what you aren’t thinking—you know that by far the simplest explanation for this unverbalizable fear of yours is just the fear of losing your fantasy of greatness, of disappointing the people who believe in you, of turning out to be pretty much ordinary, of flashing and fading like so many other child prodigies...”