Like most with a curve, mine curves left. I don't know why it's the most common curve, I've heard theories regarding left-right handedness but I would think most people would curve right then?
I read an article in a medical journal and it sounded very convincing to me. Our body is symmetrical on the outside, but internally, it is asymmetrical. The arrangement of internal organs in the chest, abdomen is such that there is more space constraint on the left side (stomach etc), compared to the right side (liver). The left testicle is naturally bigger and hangs lower as a result, leaving space for the penis to angle left. In the most natural state i.e. no influence of underwear/clothing from the beginning, the human penis angles to the left. Having talked of the angle, the curve is another topic. When leaning left (as in most cases), if, over the years, the penis is subject to pressure by underwear, will develop a curvature. Some of us (consciously or otherwise), start placing the penis towards the right in our underwear and over the years, it angles and curves accordingly. None of these is an overnight phenomenon. The usual/normal placement of the fly in usual/normal underwear (many of us use) is with an opening on the right. Boys whip out their dicks from this opening, pee and hurriedly put it back - either towards the far left, or leave it right there on the right side. Very few take the effort of ensuring it is in the center or any other position.
The article was from a perspective of majority of males, not the exceptions. I was pretty convinced about the reason for left angle and curvature, although I dont have the medical skills to vouch for it.