Question

why should we not breathe through our mouth

Answer 1

One should breathe through the nose as the nose is lined with mucus and have fine hairs. These hairs trap the dust and pathogens and don't allow them to enter the body. Also, the air gets warmed before it enters the lungs. Our mouth does not have fine hair and thus won't be able to trap dust and pathogens and prevent their entry into the body. So, we should not breathe through your mouth.

Answer 2

There are tiny hair present in our nostrils that filters the air and doesn't allow the dust particles to enter. While if we breathe through our mouth, the particles will not be removed and hence will reach our lungs.