Former dental assistant here.
Mouth breathing causes dry mouth, aka less saliva. Saliva helps to protect your teeth as it washes away harmful bacteria and whatnot in your mouth. With less saliva your mouth increases in acidity due to less saliva protecting your teeth. More bacteria=gingivitis=more decay.
Also, the nose “filters” out a lot of impurities, dust etc, so when you’re breathing through your mouth you’re also breathing more impurities into your mouth.
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