I have what's called a deviated septum, which basically means my nostrils are shaped weirdly. In my case, my nostrils are very narrow and become plugged easily, so blowing my nose is completely ineffective 99% of the time. It usually just causes my nose to become completely plugged and/or run even more. If I sniff hard and deep enough, the mucus sometimes can be driven back into my throat where I can swallow it or cough it out. But most of the time I am doomed to sniffle. My options are either sniffle constantly, or attempt to blow my nose and plug it completely, thereby allowing large quantities of mucus to run down my face because I can't sniffle anymore.

As an aside, I hate going to the dentist. Not because of the person rooting around in my mouth, but because there is often a procedure where I am supposed to breathe through my nose. 75% of the time, I cannot breathe through my nose alone, as it's like trying to breathe through a drinking straw. I just can't get enough air.