What reasons does Hooper give Elizabeth to not desert him? For what reason does he smile after Elizabeth breaks their engagement and leaves?

Mr. Hooper gives
Elizabeth two reasons to stay by his side.  First, he says, "'[...] hereafter there shall
be no veil over my face, no darkness between our souls!  It is but a mortal veil -- it is not
for eternity!'"  He promises her that the veil will not always be between them.  In life,
it will always be there, but once they are dead, he will no longer have to wear it.  In other
words, life is short (and so will his time with the veil be), but eternity is long, and there
will be no veil then.  Second, he exclaims, "'O! you know not how lonely I am, and how
frightened, to be alone behind my black veil.'" He feels very isolated as a result of his
veil, and to know that he retains the sympathy and love of one person would render his isolation
much more bearable.  

When he will not show her his face even one more time,
she leaves him forever.