Thus Spoke Nietzsche

The slave revolt in morality begins when ressentiment itself becomes creative and gives birth to values: the ressentiment of natures that are denied the true reaction, that of deeds, and compensate themselves with an imaginary revenge. While every noble morality develops from a triumphant affirmation of itself, slave morality from the outset says No to what is "outside," what is "different," what is "not me"; and this No is its creative deed. This inversion of the value-positing eye-this need to direct one's view outward instead of back to oneself-is of the essence of ressentiment: in order to exist, slave morality always first needs a hostile world; it needs, psychologically speaking, external stimuli in order to act at all--its action is fundamentally reaction.

The reverse is the case with the noble mode of valuation: it acts and grows spontaneously, it seeks its opposite only so as to affirm itself more gratefully and triumphantly -- its negative concept "low," "common," "bad" is only a subsequently-invented pale, contrasting image in relation to its positive basic concept -- filled with life and passion through and through -- "we noble ones, we good, beautiful, happy ones!" When the noble mode of valuation blunders and sins against reality, it does so in respect to the sphere with which it is not sufficiently familiar, against a real knowledge of which it has indeed inflexibly guarded itself: in some circumstances it misunderstands the sphere it despises, that of the common man, of the lower orders; on the other hand, one should remember that, even supposing that the affect of contempt, of looking down from a superior height, falsifies the image of that which it despises, it will at any rate still be a much less serious falsification than that perpetrated on its opponent -- in effigy of course -- by the submerged hatred, the vengefulness of the impotent. There is indeed too much carelessness, too much taking lightly, too much looking away and impatience involved in contempt, even too much joyfulness, for it to be able to transform its object into a real caricature and monster.

...

While the noble man lives in trust and openness with himself ... the man of ressentiment is neither upright nor naïve nor honest and straightforward with himself. His soul squints: his spirit loves hiding places, secret paths and back doors, everything covert strikes him as his world, his security, his refreshment; he understands how to keep silent, how not to forget, how to wait, how to be provisionally self-deprecating and humble.

-On the Genealogy of Morals 1:10