Well there are plenty of reasons like the other person said it could be puberty your hormones changeling it can also be you feeling guilt about something u did wrong or maybe you are thinking about mistakes u have Made
I agree with OnTheFlipSide, but I'll add a little more to it. It could be because you're either doing the same things over and over. That will make you depressed, which is why you need change every once in a while. You also need to do something perhaps. I find I am most depressed when I have nothing to do. Keep yourself busy. You may not have been depressed when you were younger but as you get older you learn that the key to stop being depressed all the time is staying busy.