I'm planning on applying next summer when I'm done my CS degree and have a bit more experience under my belt, but I would love to know what are usually the reas

I'm planning on applying next summer when I'm done my CS degree and have a bit more experience under my belt, but I would love to know what are usually the reasons of rejections? As for now I'm looking for a cofounder and a great idea, but I would love to improve on possible weaknesses I have that are import to the eyes of YC.