People who are depressed often feel very negative, especially about themselves. They often feel that all the glasses are half-empty rather than half full. They often can't feel positive about themselves either. However, I don't know the author off top of my head. Of course, they should try and learn about depression on the web.
Many good links are available.
With depression, you are not just sad, you also have physical symptoms of changes in energy, sleep and appetite, along with the loss of interest in activities, combined with the inability to experience pleasure. One of the keys is that sadness often follows an event, but depression can happen without apparent cause. This usually happens the first several weeks to several months after childbirth. It usually shows up as tiredness, lack of energy, feeling overwhelmed at caring for the child. It may be caused by some of the hormonal changes caused by pregnancy.