In the world of dreams the rules of reality do not necessarily apply. Each dream is unique and it's a direct link to the soul and spirit. Psychology provides its own theories on what dreams mean, but so did tradition and the study of dreams for thousands of years before any relevant science had developed.
Dream Meanings - Thorn
Thorns in a dream are a forewarning of minor health problems for people you love. If you are cutting thorns you will soon be relieved of your problems. If you are pricked by a thorn, you will go through a period of distress.