John Eliot Gardiner, from Bach: Music in the Castle of Heaven (Preface):
A nagging suspicion grows that many writers, overawed and dazzled by Bach, still tacitly assume a direct correlation between his immense genius and his stature as a person. At best this can make them unusually tolerant of his faults, which are there for all to see: a certain tetchiness, contrariness and self importance, timidity in meeting intellectual challenges, and a fawning attitude toward royal personages and to authority in general that mixes suspicion with gain-seeking. But why should it be assumed that great music emanates from a great human being? Music may inspire and uplift us, but it does not have to be the manifestation of an inspiring (as opposed to an inspired) individual. In some cases there may be such correspondence, but we are not obliged to presume that it is so. It is very possible that "the teller may be so much slighter or less attractive than the tale." [source] The very fact that Bach's music was conceived and organized with the brilliance of a great mind does not directly give us any clues as to his personality. Indeed, knowledge of the one can lead to a misplaced knowingness about the other. At least with him there is not the slightest risk, as with so many of the great Romantics (Byron, Berlioz, Heine spring to mind), that we might discover almost too much about him or, as in the case of Richard Wagner, be led to an uncomfortable correlation between the creative and the pathological.