Overbites cause it too. They make the mouth difficult to close properly, at least not without thinking about it. In the States, that's a mark of low income, of course, since orthodontists cost so much. I think only those on Indian Health Service get that free, regardless of income.

Just a warning, though: anything breathed through the nose goes straight to the brain, so if you find yourself someplace with a possible airborne pathogen, tie a bandana around your mouth and breathe through the mouth till you can get out of there. Do the same if you use bone meal in your garden. One sniff and you're fucked: it's made from 'downer' cattle.