You smell your dog (or anything, for that matter) when different molecules hit the olfactory sensory neurons in your nasal cavity. Dog smell -- the funky kind -- comes from yeast and bacteria that take up residence in Fido's fur. As Nick Stockton at WIRED explains, bathing your dog might make her cleaner, but it also unleashes the smelliness of whatever is living on her. From WIRED:
So what’s water got to do with it? When dry, these microscopic pieces of dung don’t emit a scent. H20 breaks down the chemical bonds that hold the micro-excreta together, releasing a fog of musty molecules into the air. As water evaporates it also raises the relative humidity in the air surrounding your dog. Humid air can hold more molecules (warm air does this too, and together heat and humidity can have a compounding effect on smell), and the higher concentration of molecules in the air means more can make their way into your nasal cavity to assault your olfactory receptor neurons.
So the next time you let your pup run through the sprinkler, just imagine all the itty bitty bits of fungi and bacteria shooting into your nose.
As the American Chemical Society points out, wet hair isn't the only nasty dog-related smell. Dog food is also pretty gnarly. That's because dogs are into the smell of molecules, such as putrescine and cadaverine -- so named for their distinct dead body smell. Your dog would like her kibble to smell like a rotting corpse, you'd like it to smell like filet mignon, and dog food chemists have to come up with something that makes everyone happy.
If you need more on dog smell, get the other side of the story in this video: Why dogs like sniffing butts.