In my case, I realised that when I was depressed, I stopped myself from doing things - I put obstacles in my way.
When you're melancholy it's like when you're in your garage and the engine won't start.
When you're content, you're revving the engine, raring to go.
To sum up, depression is when you're waiting not to do things and happiness is when you can't wait to do things.
How would you describe depression?