The change in temperature often causes nonallergic rhinitis (or runny nose to me and you). Your nose must try and heat the air that you are breathing in and to do this the blood vessels beneath the mucus-secreting glands get bigger and in turn create more mucus. Some will go down you sinus into your throat but the rest drips out your nose.
there is a build up of mucus as you have bacteria that your body doesn't want which could be the cold. It's like blood. When you cut yourself you bleed because your body is getting rid of any germs or bacteria that has got in through the cut.
Hope this answered your question :)
Do you mean when you have a cold? I believe it is because your sinuses get irritated and are more sensitive. I am not sure how the biology behind it works. So basically you produce more of the snotty stuff to keep the irritation at bay.
gud questin i have no idea.