WHY does a human only breathe through one nostril?

Roughly every 4 hours, there is a change in the "lead" nostril.

This phenomenon has a name: the nasal cycle. Namely, the two parts of the nose are able to change the resistance of the mucosa in response to the air flow passing through them. That's possible due to the special structure of the blood vessels in the nose. The cavernous body swells, blocking the passage of air, or, conversely, shrinks in size.