Why does your dog pee only on your stuff?

In all manufactured items, there is a chemical that is released in trace amounts, and is harmless to all humans and imperceptible to most humans. Dogs, however, CAN detect these minute quantities of this chemical, and it has an effect similar to catnip (in that it alters the dog's brain chemistry). This chemical's effect makes the dog think that the item in question is on fire, so they try to put it out by peeing on it.