Here's something someone who doesn't own a dog just wouldn't understand.
You see a lump of dirt on the floor of your house. You know it's been raining and you were in the yard so there's a very good chance that you tracked it into the house. You pick it up. And then it hits you. Just as you have it in your hand, you begin to wonder, "is this really just dirt?". And before you realize what it is you're doing, you lower your face to sniff the foreign substance. Just as your nostrils begin to suck in the aroma of the lump in your hand, your brain fires off an alarm warning you that it might be poop. Poop that is in your hand and inches from your nose. Breathe deeply.
It was a false alarm this time. It was, in fact, just a clump of dirt. But I have to be more careful. It's one thing to clean up some dog doo, but it's another to scrub puke off the walls.