This maxim of egoism ...

Our constant discontent is for the most part rooted in the impulse of self-preservation. This passes into a kind of selfishness, and makes a duty out of the maxim that we should always fix our minds upon what we lack, so that we may endeavour to procure it. Thus it is that we are always intent on finding out what we want, and on thinking of it; but that maxim allows us to overlook undisturbed the things which we already possess; and so, as soon as we have obtained anything, we give it much less attention than before. We seldom think of what we have, but always of what we lack.

This maxim of egoism, which has, indeed, its advantages in procuring the means to the end in view, itself concurrently destroys the ultimate end, namely, contentment; like the bear in the fable that throws a stone at the hermit to kill the fly on his nose. We ought to wait until need and privation announce themselves, instead of looking for them. Minds that are naturally content do this, while hypochondrists do the reverse.

Schopenhauer
Translated by T. Bailey Saunders