I believe the amount of melanin your skin is pigmented with is what causes the varied responses to sunlight in so many different people.
I've burned myself twice so far this summer... summer break that is
I imagine I'll burn some more before the end of this break.
Personally I think it would be great to genetically engineer my pigments, like the photosynthetic variety. After the operation I could try to scare the nurses/docs by pulling the Hulk thing.