Reverend Hooper is a complicated figure in the story because we are never quite sure why he doesn't either take off his veil or explain the veil to his congregation.
Hawthorne tells us that Hooper is "a good preacher" who tries to lead his congregation "by mild persuasive influences" rather than through fear and threats of damnation. Hawthorne is implicitly comparing Hooper to real-life Puritan minister Jonathan Edwards who wrote the famous sermon "Sinners in the Hands of an Angry God," which was the opposite of Hooper's mild persuasion. When Hooper puts the veil on, however, he essentially becomes a stranger to his congregation because they no longer "recognize" the man behind the veil.
Because Hooper doesn't explain why he is wearing the veil--other than using the theme of "secret sins" in his first sermon with the veil on--the townspeople begin to look upon him with fear. More important, even Hooper begins to look upon himself with the same fear. When his fiance, Elizabeth, for example, tries to get him to take the veil off even for a minute, he refuses, but he also tells her she doesn't know afraid he is "to be alone behind my black veil."
Even though Reverend Hooper conducts himself as he always did, he never regains the love and trust of his congregation. Elizabeth leaves him and doesn't reappear until Hooper is on his deathbed. The symbolic meaning of the veil--even though his congregation does not know what that meaning is--seems to become more important to Hooper than human relationships, more important, even, than a life with Elizabeth.
One has to question whether Hooper has essentially allowed himself to become a martyr to a symbol, willing to destroy his relationship with the rest of mankind (not to mention Elizabeth's love) in order to send a message that no one quite understands.