What is the exact reason Mr. Hooper wears the black veil?

Author Nathaniel Hawthorne writes his "The Minister's
Black Veil," as he did The Scarlet Letter and another story,Young
Goodman Brown, to expose the great flaws of Puritanism, a religion that took Calvinism
to extreme.  At the center of Puritan theology was an uneasy mixture of certainty and
doubt; and, it is this uncertainty and its resulting hypocrisy in Puritanism that
Hawthorne examines in his narratives. 

The doubt centered
on whether a particular individual was one of the saved or one of the damned.  A person
was saved by the grace of God, and would feel this grace arriving,
in an intensely emotional fashion. Although a Puritan minister, Mr. Black, as
the instrument of Hawthorne's pen, suspects that some of the seemingly righteous of the
congregation are not so.  Thus, he dons the veil to shake up the hypocrites in his
congregation, as well as to suggest his own humble being that is capable of sin, as
well.  This assault of the precepts of Puritanism and the assault upon their
consciences is more than the congregation can bare.  Some feel guilt and turn away,
while the more stalwart hypocrites attack the character of the minister himself, whose
primary guilt is Hawthorne's guilt:  shame for the sanctimonious hypocrisy of
Puritans. 

The ambiguity of the veil is at the center of
the theme of Hawthorne's great story, which he calls "A Parable."  For, it underscores
the lesson of his greatest work, The Scarlet Letter, in which he
exhorts his readers,

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Be true!  Be true!  Show freely to the world, if
not your worst, yet some trait whereby the worst may be
inferred!

As he lies dying,
the Reverend Mr. Hooper refuses to remove the dark veil from his face. He raises his
trembling body and speaks,

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'Why do you tremble at me alone?....Tremble also
at each other!...When the friend shows his inmost heart to his friend; the lover to his
best-beloved; when man does not vainly shrink from the eye of his creator, loathsomely
treasuring up the secret of his sin; then deem me a monster, for the symbol beneath
which I have lived, and die!  I look around me, and lo! on every visage a
Black
Veil!'