I am 20 and I have cellulite on my thighs and a little on my butt. What is the reasoning behind this? I don't have kids and I weigh around 130.

No reason. Usually cellulite occurs somewhat older, but there is no way to prevent or really treat it. Stay in good condition with movement and resistance training. Eat real food, like fruits and vegetables. Along with protein. It's a normal occurrence and there's not much you can do about it. Don't let dimpling of your fatty tissue bum you out.