If you feel like you bottle up your emotions a lot, you could try really opening up to another friend, it definitely helps get stuff off of your chest. Also, know that a LOT of people care about you even if they don't always tell you.
Try doing more things that you enjoy, treat yourself a bit and try eating healthy and getting a good amount of sleep and exercise. These can help so so much don't underestimate what a good nights sleep can do.
You said that you don't know why you're depressed, so I think (from personal experience) that you might just be used to the sadness. Have you tried taking any medicine?
Anyways, if anything you can talk to me about anything on your mind if you're bored or just need someone to talk to