It boggles the mind that so many threads related to masturbation/arousal can occur at the same time...
My answer is because it feels good. The physical and psychological release can relief minor and often major stress points, as well as alleviate any tension amongst the blood vessels (that CAN be alleviated, that is). Not only that, but the metaphysical and emotional connections can strengthen ideas and ideals together. It's the main reason we see sex as the number one selling tool, because, inherently, all mankind has the ability to satisfy that urge, be it with another or alone. They are vastly different situations, of course, but the purely physical response can be recreated. The mental side of it all can make the most difference... Is it for boredom? Are you bored? You can do it like that. Are you lonely? You can feel like there's someone important trying to cheer your demeanor, or you can imagine anything you want and go nuts (lol) with it all. If the idea is simply to let loose and give yourself exactly what you desire, then it is there. We do because we want to and we can. And it'd be waste NOT to rub one out for the little guys (for some of you, that may be a mixed message...).
Ladies (I'm in that group), it's merely because we CAN. Everything else is trivial.