Suppose that I haven't taken a shower for a while, or haven't disposed of my trash, or something, and the room I've been in now stinks. Then, to remove the smell from the room (presumably after first throwing me out of it), you can just open a window or door, and let in the fresh air. Intuitively, the reason this doesn't leave a permanent stench outside, is because there's a lot more atmosphere outside than inside, thus diluting the smell so much it becomes unnoticeable.
But wait! There are billions of people on the planet, and life has existed on the planet for billions of years. And surely, most of the lifeforms that have existed here have made lots of bad smells, perhaps especially after they're dead. (And I sure don't think dogs smell good when they're alive either, for that matter.)
And yet, we call the air fresh, for the most part. So I suppose that, over time, the bad smells must have been destroyed or absorbed somewhere, and now I'm curious as to where. Would you have any ideas?