What I feel

So I have been suffering depression for about 5 years,with other issues as well, but I don have a reason too.  I have a great family, good school system, though I disagree with the national schooling system, and my life is pretty good; but I still feel depressed.  I hide my depression from everyone, and so no one knows, but on the inside I feel lonely, bored, empty,and I feel like there is no purpose.  So if my life is good, why am I so depressed?