I used to think my former drill sergeant was the meanest man alive when I was going through basic training, but I realize now he was preparing me for the worst conditions possible. Real combat is confusing and stressful, and so is Army basic training. The drill sergeant barks out orders the same way a superior officer might do it during a real fire fight. It's not personal, although it can feel that way during basic.
I couldn't wait to graduate from basic and move on to my assigned training school, mostly because my drill sergeant rarely gave me a minute's rest. I understand now why he did it, and his voice in my head probably saved my life when I was deployed into a combat zone two years later.