What is with Mr. Hooper's sad or melancholy smile? Hawthorne mentions this many times. Why is he smiling like this and so frequently?

In Nathaniel Hawthorne's "The Minister's Black Veil", Minister Hooper's sad or melancholic smile possesses many interpretations due to the nature of the story and the time it was written, so it doesn't hold a sole or exclusive signification. Among the different alternatives, there is:

1. The sadness of being misunderstood on the message that he was trying to get across the people of the town (The idea was that everyone wears a veil - hiding behind a facade - to keep a secret or sin from society). The difference is that he did it on the outside instead that on the inside.

2. How people opted to fear, avoid or question him as soon as he started to wear the veil.

3. While he tried several times to explain the motives behind his veil wearing, the attempts were unsuccessful and after a while, he just chose to smile instead of trying to make his parishes understand his actions.