The more you feel this way, the greater the stress. Frequent occurrences of negative mood indicate that stress is affecting you detrimentally. You may be experiencing personal burnout. You may also be trying to communicate with others in an indirect way that you are unhappy with yourself, them, or life in general. Unfortunately, they may not be able to interpret your moods, nor know what to do.
Some negative mood is normal.
It cannot and perhaps should not be avoided. Occasional down times may have a cleansing effect. For example, after being rejected for a job promotion, you feel down and blue. This is a normal reaction to your perceived failure. As you work though your down feelings, you come to realize that there are things you can do that will improve your situation, such as move, get a new job, or simply talk to your supervisor about what you can do to improve yourself.