Why is my 3DS pad drifting?

Go to settings, and you should find buttons for calibrating the screen using a stylus, and one for calibrating the circle pad, the circle pad is the one you want. it should be somewhere near where date and time is. If the stick moves on its own (the physical part, then I recommend opening it using an ifixit guide, and looking at the circle pad to see if it is dirty, and if it is, then take it out and clean it.