Unsolved life problems
Accumulated problems and unattained goals will create bad moods once stimulated or being remembered by a certain incident. For example,someone who can`t make enough money is most likely to avoid certain topics that are related to money or someone else`s success.
The larger the time gap between thinking about a certain action and actually taking that exact action the more likely for bad mood to be established.
Depending on an external source for stimulating dopamine
Substances like opiates or crack cocaine stimulate the central brain causing the production of -above normal- levels of dopamine and endorphin responsible for feeling good. Any absence of the drug will cause the user to feel down and have severe mood swings. (See What Alcohol does to your Brain?)
Remembering a bad event
Any incident or stimulation that will result in reminding the person with a bad or an unwanted memory will affect this person`s mood. Feeling bad, slightly bad or very bad however will depend on how strong the stimulus and how bad the memory or the event remembered.
Pre-Menstrual Syndrome (PMS)
Though the degree of severity will defer from one woman to another but the majority of women will suffer a change in the mood right before their monthly period.
When a person realize that he/she has been wasting time
Spending too much time on social Media or TV then realizing that it was a complete waste of time is one of the reasons that can cause a bad mood, especially when being subjected to a high level of negativity both online and on TV.
Bad eating habits and negative self image
Unhealthy life style and not being satisfied with one`s looks can cause someone to feeling bad when comparing him/herself to someone who`s better looking or right after being full and realizing that this is not the right way to approach eating.
Incidents or events that stimulate fear or worrying will cause a negative change in s person`s mood when if left without being dealt with or when repeatedly being focused on. Repeatedly checking the email for an important mail can cause worrying, asking questions and eventually a person to feel bad.
Going on a no-fat diet or reducing carbohydrate to quantities that are lower than normal will result in physical and mental fatigue beside, the feeling of deprivation and not being able to maintain the same eating behavior for a longer period of time can cause the person to feel bad.
Lack of proper sleep or staying late for a long time will cause mind fatigue, bad temper, lack of patience and aggression which will eventually affect the person`s overall mood. (See How does staying up late affect your health?)