This common and unfortunate fact of the lack of an adequate presentation of basic ideas and motivations of almost any mathematical theory is, probably, due to the binary nature of mathematical perception: either you have no inkling of an idea or, once you have understood it, this very idea appears so embarrassingly obvious that you feel reluctant to say it aloud; moreover, once your mind switches from the state of darkness to the light, all memory of the dark state is erased and it becomes impossible to conceive the existence of another mind for which the idea appears nonobvious.
— Gromov

Source: M. Berger, Encounter with a geometer, Part II, Notices Amer. Math. Soc. 47 (2000), no. 3, 326--340.
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http://www.ams.org/notices/200003/fea-berger.pdf