Mr. Hooper's donning of the black veil might suggest the idea that people can view others by filtering their perceptions of friends and strangers through their personal knowledge of sin. 

On the other hand, Reverend Hooper may desire that his congregation look inward to their own souls. When the minister gives his sermon, 

Each member of the congregation. . . felt as if the preacher had crept upon them, behind his awful veil, and discovered their hoarded iniquity of deed or thought.

The ambiguity surrounding the veil causes the congregation to also consider their own sins. When his fiancée questions Mr. Hooper about his reasons for wearing the veil, he suggests two possibilities:

"If it be a sign of mourning," replied Mr. Hooper, "I perhaps, like most other mortals, have sorrows dark enough to be typified by a black veil."

"If I hide my face for sorrow, there is cause enough. . . and if I cover it for secret sin, what mortal might not do the same?"

By wearing the veil and creating ambiguity, the Reverend Mr. Hooper hopes that his congregation may become more understanding of others. He also hopes that they become aware that their judgments of others may be influenced by their own guilt and knowledge of sin. He also hopes that they will examine their own consciences. 

While there is some implication within the story that Mr. Hooper's decision to start wearing the black veil had something to do with the death of a female member of his congregation, we never learn for sure exactly why Mr. Hooper chose to wear it. Ultimately, this uncertainty helps to emphasize one of the story's primary themes: that human beings are inescapably alone and isolated from one another. On the day that Mr. Hooper starts wearing the veil, he presides over a funeral for a young a female member of his congregation. When Mr. Hooper leans over to view the body, he accidentally lets his veil slip and comes face to face with the deceased. Later on, several members of the congregation claim that they saw the spirits of the woman and the pastor walking hand in hand during the funeral procession. Together these two incidents imply some kind of significant connection between Mr. Hooper and the young lady. This possibility also feeds into the suggestion, voiced by many throughout the story, that Mr. Hooper wears the veil as a result of some secret sin or indiscretion. While it is repeatedly suggested that there is some connection between the pastor and the young lady, these suspicions are never confirmed and Mr. Hooper's choice remains a mystery. This mystery makes it hard for the reader to fully understand Mr. Hooper's thoughts and actions or to truly identify with him. This inscrutability emphasizes the loneliness and solitude of the pastor and, by extension, drives home the point that we can never truly know and understand one another.

We never get to find out exactly why Mr. Hooper wears the black veil, which merely adds to its mystery. Nevertheless, there are one or two subtle hints in the text that point towards a possible answer. When his fiancee, Elizabeth, comes right out and asks him why he insists on wearing the veil, Mr. Hooper's response is cryptic:

"If it be a sign of mourning," replied Mr. Hooper, "I, perhaps, like most other mortals, have sorrows dark enough to be typified by a black veil."

Precisely what those dark sorrows are we do not know, and Hooper certainly doesn't care to elaborate. But at the very least we can surmise that the reverend has experienced a good deal of sadness in his life worthy of mourning. And as a black veil is traditionally a sign of mourning, he feels it entirely appropriate to wear one, even if it scares and unnerves virtually the whole of his congregation.

Even on his deathbed, Mr. Hooper refuses to spill the beans. But once again, there are hints. The suggestion is that Mr. Hooper harbors a secret sin that is a matter between him and his God. It would therefore be inappropriate to divulge what causes him to wear the veil. As he looks around him, everyone else appears to be wearing a black veil—i.e. has their own secret sin—but unlike Mr. Hooper they are not prepared to acknowledge their faults, choosing instead to castigate someone for wearing an unusual item of clothing:

"Why do you tremble at me alone?" cried he, turning his veiled face round the circle of pale spectators. "Tremble also at each other! Have men avoided me, and women shown no pity, and children screamed and fled, only for my black veil? What, but the mystery which it obscurely typifies, has made this piece of crape so awful? 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!"

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,

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,

'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!'

Hawthorne never explicitly gives us a reason for Hooper's wearing of the black veil. Even when other characters in the story, including Hooper's fiancee, inquire as to the reason he wears the veil, Hooper never answers. This is part of the attraction of the story. Each reader is able to create his or her own specific reason.

However, from examining several details within the story, we can infer at least some basic ideas. First of all, when Rev. Hooper appears wearing the veil, his sermon that day is on the idea of "secret sin," sins of which each individual is guilty but that we never reveal to anyone. It can be inferred from this that Hooper too is guilty of secret sin. Later that same day, Hooper attends the funeral of a young maiden. The townspeople suggest that Hooper's spirit and the maiden's spirit are connected in some way, and we can infer from this that perhaps Hooper's secret sin has something to do with this young maiden. However, no further detail is provided and each reader is left to fill in the gaps of information.