From Alexander Schmemann, Eucharist (page 186, I think):
Not giving thanks is the root and the driving force [of pride].... It alone is not from below but from above: It is not from imperfection but from completion, not from deficiency but from an overabundance of gifts, and not from weakness but from power. In other words, it comes not from some unexplainable 'evil' of an unknown origin, but from the enticement and temptation of the divine 'very good' of creation and man. Pride is opposed to thanksgiving precisely as unthanksgiving because it arose from the same causes as thanksgiving. Is another, opposite answer to the same gift; it is temptation by the same gift.