Our skin is a constantly changing and traveling planetary surface of innumerable species, tribes, and climates. So are many of the tissues inside.
A recent XKCD riffed on our visceral terror about this.
Not only are our bodies staggeringly complex and ever-changing ecosystems with an astounding range and quantity of inhabitants, but the health and stability of those systems is key to our health. We need those stomach flora and scalp residents and all the rest, if only to prevent having those warm, wet, nutrient-rich territories colonized and stripped by species that we're not in symbiosis with. And yet much of our "health care", from mouthwash to how we use antibiotics is premised on the brutal and flawed logic that the best way to address a bodily problem is to do our best to kill all the microorganisms that live in that part of our bodies. And that way of looking at the world informs our approach to everything from how we treat "weeds" near our homes to how we treat people living under bridges. If we can just finally find the right collections of toxins and attacks, we tell ourselves, we can keep our world "clean". Though it rarely works for very long, does it?
We endlessly fight inevitable equilibrium states at enormous cost and view the world from a constant adversarial place, in our tensely defended bastions, with trimmers and toxins always kept close, ready for use to kill anything we didn't put there. Have you ever met a homeowner who didn't think of the living things around their home as an enemy, to one extent or other?
What would it do to our definitions of self, cleanliness, and ecosystems if we all lived our lives aware that we are what we are? That sterility is death.