With the chemical changes in the brain, this can cause lethargy, lack of motivation, etc. so some of it is just a natural physiological response. Also, having depression can make someone start to neglect themselves and their own needs. I know when I feel that way, I begin to feel "what's the use?" and "nothing can help me feel better." Then, that leads to giving up on even trying to do activities that are typically enjoyable when not depressed. I begin not to care too much about myself, so I stop taking care of my health. Also, I think inward pressure to "feel better" can be so overwhelming that it can lead to a sense of powerlessness, that nothing is worth trying anymore.