It may be hormones or as a result of your surroundings. Sometimes, children with anger-filled parents tend to grow up in anger and they do not know how to control it. Too much anger can do a lot of damages and it is important to control it. Learn how to control it at http://www.apa.org/topics/anger/control.aspx.
It's normal to have teenage mood swings, what with spiking hormone levels and your body and mind working to master this maturing new self. But prolonged 'very mad' could be worrisome and a symptom of something else, clinical depression for example. A big part of our teen years are spent learning to control ourselves, developing self esteem and personality, and a large capacity for having fun. If your 'mad' feelings are interfering with this natural emotional growth then it's time to get help from an adult you trust.