I don't think any one can truly understands some ones depression. For every one it's different (or at lease how I see it) and can't just be easily explained to some one. For most people they don't even know why they're depressed.
No one can really understand how I feel and I don't expect them to ether. I cant even understand my own depression. I do really dislike it when people try to say they understand though. I know their just trying to be nice but after hearing it so many times it makes my head want to explode. I also always get the "Your not depressed" thing from every one. I think that one's even worse.