In addition to the dark and gloomy tone pointed out by other commenters, this story is also characterized by an immense sadness. Hawthorne validates Mr. Hooper's belief that each of us hides a secret sinfulness that we want, at all costs, to prevent others from suspecting; Mr. Hooper believes that we not only hide our true selves, then, from others, but we also try to hide from ourselves and even God. When Mr. Hooper gives his first sermon wearing the veil, his congregation seems to acquire some inkling of its meaning, and this causes them to distance themselves from him. It is too painful for them to think that he might have an accurate understanding of their souls. Further, when Elizabeth, Mr. Hooper's fiancee, figures out what the veil symbolizes, she blanches and runs from him, never to return again. These responses seem to show that Mr. Hooper is correct. And if he is correct, then this means that none of us ever truly knows or is known by any other human being, including, perhaps, ourselves, and this is an incredibly sad and isolating notion. In his final moments, the parson says,
"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!"
Therefore, since Hawthorne validates Mr. Hooper's behavior, it means that Hawthorne must, on some level, relate to Mr. Hooper's terrible sadness as well. The idea that we are all trying to hide our true selves means that we are all, voluntarily, alienating ourselves from everyone else for the entirety of our lives. Our need to hide the very thing we all have in common means that we can never really know anyone else or be known by them. This terrible irony is tragic and helps to produce the story's sad tone.