Why do some people get colder than others?

I went to the beah today in poor weather and went in the sea wih my friends. We were in the water for about 20 mins with no wetsuits. After we got out and dried off all my friends were fine but i ended up with moderate hypothermia and took me about two hours to recover. My friends are all the same build as me if not skinnier so why did i end up with hypothermia and they were fine?