Why should we only breathe through nose

Because it makes it easier for the air to flow through our bodies.

There are many reasons but to name a few:

-prevents respiratory problems such as asthma, hyperreactive airway, shortness of breath, etc.

-good for digestion

-prevents snoring

-the nasal hair filters out chemicals from the air, while the mouth has no filter.

-there is a mucous membrane that can kill pathogens, especially airborne ones

many more