Why do you get a cold sore?

Stress, exposures to. Some will get them from excessive exposure to sunlight, others to tomatoes, others to spicy foods. By far the major contributor appears to be stress.
Herpes Simplex virus. Cold sores are painful liquid filled blisters that can occur on the lips, roof of the mouth or the gums which are caused by the herpes simplex virus (hsv-1). An estimated 80% of americans have been exposed to herpes simplex virus type 1. However, not all of those people will get cold sores. It depends on your immune system.