How do refrigerators get so nasty inside?

Seriously, what happens inside when the doors are closed? I just don't understand how they get so nasty? I try to clean it out regularly, but everytime I'm amazed at the crap I get off the shelves! I think I can blame the nasty hot dog juice, that the kids always seem to dump, for the mess in the meat drawer! lol It's always something, isn't it?