The arm angle has a lot to do with it. I was eleven years old when I discovered that I had a natural sidearm delivery---and a good curve ball that came attached to it. I quickly found out that the sidearm delivery is the easiest on the arm and shoulder, and I had no problem when it came to breaking pitches---good thing, too, because I wasn't fast, not by any stretch of the imagination, and had to go to the "snake jazz". A little later I found an incredible pitching coach who helped me refine my stuff, and the best part was that he did not mess around with my delivery---he firmly believed that every pitcher has a natural motion, and what he did was show me how to make the most of it.
What really gets my goat is when I read about young pitchers who have all kinds of arm problems because they're forced to do such things as---pardon the expression---throw over the top, in spite of the fact that it may not be their natural delivery. More than once I have read about a kid who gave up on the game because he was being forced to use a delivery that did not feel comfortable for him, just because a parent or a coach wanted him to do it and would not stand for anything else! So when I find out that a kid is using a pitching delivery that feels right and is comfortable for him I advise him to stick with it.
One more thing, and this I picked up from watching how the Yankee pitchers used to do it---I noticed that they were driving off the lower half of the body, using the legs, the hips and the torso in a continuous, seamless motion to generate the power behind their pitches, and that doing this took a lot of pressure off the arm and the shoulder. This is an essential aspect of mechanics, and if more pitchers would learn to do this it would prevent a lot of problems. I picked up on this, and my coach helped me refine it the way he did with everything else. 8)