Why does Mr.Hooper flee from the reception?

Mr Hooper caught a glimpse of himself in a mirror.

"At that instant, catching a glimpse of his figure in the looking-glass, the black veil involved his own spirit in the horror with which it overwhelmed all others. His frame shuddered, his lips grew white, he spilt the untasted wine upon the carpet, and rushed forth into the darkness. For the Earth, too, had on her Black Veil."