Their theory is that it's just your brain's attempt to get you warm. We do, after all, think of romantic feelings as getting us "hot" (moist palms, blushing cheeks, an accelerated heart rate). It seems like you actually get even warmer by running through the desert from a chainsaw murderer, but whatever. We also associate closeness with other people with "warmth" (if you say relations between two people have grown "cold," you sure as hell don't mean they're hugging a lot), so when you get cold, you want to watch other, sexy people steadily getting closer for 90 minutes.
Our brains subconsciously tell us that we need heat during cold weather and make us crave this psychological "warmth." Unfortunately, achieving this requires being repeatedly subjected to Katherine Heigl and Ashton Kutcher, but hey, no one ever said survival would be easy.
"We may be stranded in these woods, but Love Actually on my iPhone will keep us warm."