In the movie he says, "This face infection which ruins our love." I can't remember the reason for it in the book though.

Yeah he was born that way

I was under the impression it was a birth disfigurement.

In the book it says he was born that way and his mother never kissed him or anything. He had a sad life

honestly! It reminds me of the musical Wicked.. Cuz she was born green and hated....

i believe it is was an infection caused at birth.

Yes, he was born with it. He even says it in this film: "This face that earned a mother's fear and loathing, a mask, my first unfeeling scrap of cloathing."

he is not ugly also without his whithe mask

I think he was just probably out in the sun too much the day before.. Because really in "stranger than you dreamt it" his face is fine ... I think he wears the mask to look cool..

(that was a joke) The answer has been answered several times, I can't add to it.. But really the evidence does all point to sunburn..