Day to day ups and downs are a normal part of adolescence, making it difficult to distinguish between normal teenage moodiness and depression.
Depression builds walls around people and between people. When someone you love has been dragged inside those walls, there can be a distance between you both that feels relentless. You miss them, but they’re right there beside you, except that they’re kind of not. Not in the way you both want to be anyway.
When depression latches on, it settles in and rearranges you to get a better grip. You stop loving the things you loved. You stop looking forward to anything. You feel hopeless and you feel sad.
In the Western world, 1 in 10 people will suffer depression during the course of their life. It has more of an effect on physical health than diabetes or arthritis.