Imagine if you will a moment in your life when you were struck dumb by the awe inspiring beauty of something: the green flash of a sunset, the stillness of the quiet woods, the first snowflake of winter, what have you.Image you experience that every day.
Then, after a while, the world shifts to tones of grey... sounds become flat, and every time you look at that sunset sorrow washes over you. You know that it's beautiful, but you can't feel it any longer. That's depression.
Your entire frame of emotion is shifted down. The lows are deeper, the highs aren't as high, and you remember when it wasn't this way.
And there is a deep seated intuition that you are emotionally "flawed" somehow.