R. Hershel Schachter in this week's TorahWeb article:
Chava sinned with the eitz hada'as because she fooled herself into believing the words of the nochosh, that she "will become as great as G-d Himself." Many individuals are led to sin because they fool themselves into believing that they are someone other than who they really are; they join groups of people where they don’t really fit in. To fool others is a serious sin; to fool oneself is a greater sin.