Do you think your parents love you? Do they show you? Do they give you the things you need? If your answers to these questions are yes, then seeing you do things that are not good for you (whether you understand/believe this or not) pains them terribly. Unfortunately there are people out there for whom it is difficult to express their emotions and show love to their close ones. I am sure such a situation would bring them pain as well, but they may not be able to show or express it easily or at all.
I’ll give you a small example. Two boys were playing and one of them ran in the street and was almost hit by a car. Two men were walking close by. One man continued walking and the other ran over and screamed at the boy for several minutes. Why do you think he screamed at the boy? Because he’s his father. A stranger could hardly care less and even an acquaintance would not feel comfortable disciplining the boy. Only his father, out of his tremendous love for the boy, who became so frightened when he saw his son almost killed by the car, chastised his son and made him understand how important it is to be careful near the street and cars.
I hope this story makes it a little clearer. Even if your parents act in an angry way, there is a motivation of love. It may be deep down and there may unfortunately be other emotions within them, but they do love you and it does pain them to see you doing things that will hurt you in the long run.