A manic depression is different from a normal depression in the fact that a person not only has periods where he feels very sad, he also has periods of time where he feels himself abundantly well. Although this may sound good, this is not the case as people loose reality of who they are and what they can do. Some people for instance think they are superman or god and will act in the same way.
Their are many possible causes that are the root of this order but the truth is we do not know exactly how a disorder like Manic Depression is caused. However, from what we do know scientific research has shown that there is a genetic link. The incidence of this disorder is higher among patients where their father or mother also had manic depression. This does not explain the whole process why some people do and others will not get this disorder.
The threshold theory says that everybody has a chance on manic depression, but for one person the chance is much higher than for the other. If you will get a certain disorder like manic depression would depend on the amount of stressful life events. So this theory states that it is a combination between sensitivity and stressful life events.
Although this theory has some very strong points, it also has some flaws as it is a very general one not pointing to the very complicated structure of the brain. More research is needed on this topic before we can fully understand what are the causes of manic or bipolar depression.