Either extreme is bad, IMO.

Too much, and it's all you're doing. Masturbation is subject to the law of diminishing returns, like any other pleasure. The more you do it, the less pleasure you get from it, the more you need to do to get the same amount of pleasure, etc.

No masturbation isn't natural. It's denying one of the most important aspects of life, and all that built-up sexual tension may cause you to get too uptight and cranky with people.

I think the key is to find a balance. "All things in moderation," as one of my high school teachers used to always say. Don't use up all your energy in masturbation, or you'll never get anything done. Likewise, allow yourself to relax once in a while and enjoy a harmless pleasure.