Why do people do drugs?

I don't understand why people used drugs. It will kill your brain cells. My brother use drugs since he was really young. He hang out with the wrong crowd. Most of our friends was using drugs. I remember when I used to go clubbing with my friends. They was so high they didn't even know what they were doing. One of the girls was so high she got knock in the face. She didn't even know she was bleeding. I still don't understand why!