In the story, Mr. Hooper's fiance begs him to take off his veil. She states that the veil hides the face that she is always glad to look upon. However, no matter how much she protests at his strange behavior, Mr. Hooper refuses to put aside his veil. To make matters worse, he answers her cryptically:
"There is an hour to come," said he, "when all of us shall cast aside our veils. Take it not amiss, beloved friend, if I wear this piece of crape till then."
What he means here is that there will be time enough to reveal one's secrets when everyone stands before God in judgment. Here, he is referring to the Christian doctrine of the hour of judgment, when every motive and action will be made plain. Until then, he admits that he is forever obliged to wear the veil without the right to fully disclose the reason why. When his fiance begs him to stop being so mysterious in his speech, he tells her:
"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 No mortal eye will see it withdrawn.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!"
Here, we see that Mr. Hooper has somehow made a sort of vow to wear the veil. Apparently, this vow also constrains him from divulging the secrets of his strange habit. It is a sad state of affairs, because we soon come to realize that the vow causes Mr. Hooper extreme loneliness and leads him to fear for his future; despite his suffering, however, he is still obligated to wear it. He is a man trapped beyond his ability to extricate himself from the mental agony of his actions.
"...O! you know not how lonely I am, and how frightened, to be alone behind my black veil. Do not leave me in this miserable obscurity forever!"