thinking about futere usually make us stressfull and thinkng about past make us sad , we need to live at the moments 
Um, of course "Being sad is not an illness."
I think the real depression is not a feeling. It's not being sad after a break up. It's a chemical imbalance in your brain that makes you unable to think happily. Of course no one has a reason to be depressed, that's the point. That's what makes it an illness. The fact that you're unhappy no matter what.