Your nose runs when you have a cold because your body produces extra mucus as part of your immune response. The cold virus triggers inflammation in the nasal tissues, causing your nasal glands to produce more mucus. This excess mucus helps trap and flush out germs and irritants, protecting your respiratory system from infection. The mucus buildup then drips out of your nose, resulting in a runny nose.

Additionally, the mucus serves as a protective layer to keep nasal tissues moist and prevent infection by carrying away infectious agents and irritants. This is a natural defense mechanism of the body during a viral infection like the common cold.