Three things about the sergeant-major which make the reader more likely to believe (or at least accept) his story about the curse."The Monkey's...

In W.W. Jacobs's horrifying tale, "The Monkey's Paw," the
reader is presented with a man who has been in foreign lands, a man who speaks of "wild
scenes and doughty deeds," a man who knows wars and plagues and strange peoples. 
Reasons for the reader to grant him credibility regarding the curse of the monkey's paw
are the facts that

• this sergeant-major seems
  worldly and wise, indeed, and has seen the fakirs and been in the old temples of
  India


• he is a military man, and, therefore, seems
  trustworthy


• he is a personal friend of Mr. White, having
  worked in the warehouse where Mr. White has worked.


• he is
  reluctant to talk of the monkey's paw to his old friend.  Then, when asked if he had his
  three wishes granted, the sergeant-major blanches and replies, "I
  did."


• when asked if he could have another three wishes,
  the sergeant-major then throws the monkey's paw into the fire, and he tells Mr. White,
  who retrieves it, "Better let it burn."


• The
  sergeant-major warns the Whites of the consequences of using the monkey's
  paw.