The truth is, the reader never truly finds out why Father Hooper is wearing the veil. It appears quite suddenly, and the members of his church are most definitely put off by its addition to his clothing:
"I can't really feel as if good Mr. Hooper's face was behind that piece of crape," said the sexton.
"I don't like it," muttered an old woman, as she hobbled into the meeting-house. "He has changed himself into something awful, only by hiding his face."
"Our parson has gone mad!" cried Goodman Gray, following him across the threshold.
Hawthorne sets up this same curiosity in the reader. Most characters refuse to ask him why he's wearing the veil; rumors abound about what is under the veil and why he is wearing it. Finally, his fiance does ask why he is wearing it:
"Elizabeth, I will," said he, "so far as my vow may suffer me. Know, then, this veil is a type and a symbol, and I am bound to wear it ever, both in light and darkness, in solitude and before the gaze of multitudes, and as with strangers, so with my familiar friends. No mortal eye will see it withdrawn. This dismal shade must separate me from the world: even you, Elizabeth, can never come behind it!"
"What grievous affliction hath befallen you," she earnestly inquired, "that you should thus darken your eyes forever?"
"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."
Eventually, on his deathbed, Father Hooper gives his most clear answer as to why he is wearing the black veil and says:
"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!"
What he shares with those around him is that the veil has taken on the symbolism of the secret sin that all people have. Additionally, this small piece of crepe paper has created such an intense barrier for most people of his congregation; they were unable to see past the piece of paper, unable to focus on the quality and character of the Father. Father Hooper may have started wearing the black veil in mourning for his own sins, but it becomes representative of the dark, secret sins of all and our abilities to recognize faults in ourselves and others.
Unfortunately, Nathaniel Hawthorne never gives a black and white, concrete reason as to why Father Hooper wears the veil - the reason remains shrouded, just as the face of the Father, who wears the veil even in death.
At the end the story, why does Father Hooper look around at the Spectaors on his death bed cry, "lo! on every visage a Black Veil"?
The reason why Father Hooper wore a veil throughout his life is revealed when he is on his deathbed. Everyone in the town assumed he wore the black veil because of some sort of sin he committed. However, Father Hooper ends up explaining he wears the veil not to cover his own sin but to remind him of the sin that is in everyone. This further reinforces symbolism of the black veil as a symbol of the darkest and deepest sin that every person holds.