If you've already tried it, you'll know the answer to this question! As well as feeling good, masturbation relieves sexual tension which builds up naturally over time. Every man and boy in adolescence sometimes feels aroused or turned on, and sexual thoughts and desires seem to take over your mind! This is a natural process: your sexual organs produce and accumulate semen, and your body responds by wanting to ejaculate the semen in order to make room for more. One of the ways this happens is by nocturnal emissions or wet dreams, which happen when you are asleep.
In spite of what you might have heard (either from friends or adults) masturbation is completely safe. It does not affect the size of your penis nor your ability to have an erection, or to have children later in life. And you never run out of semen, either!