Why do we breathe through one nostril?

No idea.

Our nostrils have specialized tissues.

At a given time, these tissues are swollen in one nostril while they are shrunk in the other.

Hence, when we breathe, air passes easily through the nostril in which the tissues are shrunk.

However, due to the swelling, very less amount of air slowly passes through the other nostril.

Thus, it seems as if we breathing through one nostril only.

However, after sometime, the swelling and shrinking in the nostrils reverses.

This breathing pattern is called nasal cycle.

Nasal cycle is beneficial for us.

But how?

Some odors are detected better in fast-moving air, while others in slow-moving air.

Thus, with fast moving air in one nostril and slow moving air in the other.

Our nose can detect a greater range of smells.

Besides this, our nostrils are moist.

Continues breathing can make them dry.

Hence, when we breathe through one nostril, the other nostril gets time to become moist again.