What explanation does Othello give as cause for Desdemona's affection for him in Othello?

It is worth noting the context in which Othello must give
explanation for Desdemona's affections.  He is a very high ranking military figure, sort
of a foreign celebrity in Venice ( he reminds me in some ways of Lafayette's popularity
in the US after the Revolutionary War), wined and dined in the best of homes as an
honored guest, including the home Desdemona and
Brabantio.

All this notoriety is worth noting, because,
Othello is accused before the Duke of Venice by Brabantio (his former welcoming host) of
bewitching Desdemona so that she is made to act against her will and love and marry
Othello.  Brabantio says:

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I therefore vouch
again,

That with some mixtures powerful o'er the
blood,

Or with some dram conjured to this
effect,

He wrought upon
her.

Against which charge
Othello, this amazing celebrity, must defend himself.  Beginning with the
words:

[A]s
truly as to heaven

I do confess the vices of my
blood,

So justly to your grave ears I'll
present

How I did thrive in this fair lady's
love

And she in
mine.

What follows is one of
the most oft quoted speeches in Shakespeare's canon (I, iii, lines 128-169), in which
Othello describes (as mentioned in the previous answer) how he wooed Desdemona ending
with the words:  "This only is the witchcraft I have used."