usually it's somebody that has no clue overtightning the hell out of it, generally theres some kind of seal rubber o-ring in a slot in the plug or copper/aluiminum washer that does not requier a **** load of force to get it to seal up, id get a new plug and remove the old plug with a pair of large vice grips, get the grips on good and tight and wack them with a hammer/block of wood, a sharp quick movement will be more productive then a slow long pull, if the vice grips fail a small monkey wrench with a cheater pipe on it would be my next try, get a good bite and give a good strong pull