Why is John McEnroe known for poor sportsmanship?

While MacEnroe has latterly become an astute and humourous commentator of the game of Tennis, at the peak of his professional game he was regarded as argumentative and tempramental. Hardly a game went by with a disputed line call resulting in a "You can not be serious" or "I want the referee". However, history has rightly judged MacEnroe kindly as a true master of the game and a breaker of the Bjoen Bordom that over-whelmed the game in the late 70s.