As defined by PubMed Health:
Depression may be described as feeling sad, blue, unhappy, miserable, or down in the dumps. Most of us feel this way at one time or another for short periods.
Clinical depression is a mood disorder in which feelings of sadness, loss, anger, or frustration interfere with everyday life for an extended period of time.
The question then becomes when does it begin to interfere with everyday life and why does this happen to us sometimes and not others?
My answer is that depression is more than just feeling sadness, loss or anger. It is NOT feeling these emotions. It is the long battle to not feel. It is the exhaustion after the long battle. It is the helplessness we feel after we have lost the fight not to feel.
Depression feels like defeat, because it is.
We have lost because we have given up in a sense.
We just can't fight the emotions anymore.
And we aren't really willing to do the work to truly feel them either. We either don't know how or we don't believe we have the energy.
Maybe not consciously-could be unconsciously without awareness.
Common “depressing” thoughts: I can't do this anymore. I just don't care. Life is awful. It's not worth it. I don't have the energy.