Cloudscape

In a person who is bipolar, hypomania and depression both have the same cause and cause one another. Bipolarity is usually caused by with an increased sensitivity in some form or other, though increased sensitivity does not necessarily lead to bipolarity. As such, the bipolar cycle is merely an exaggeration of a normal person's emotional cycle between normal sadness and normal happiness.
However, at the same time, hypomania may cause depression and the other way around, as yin and yang usually do, much like the crests and troughs of the waves. For one thing, hypomania may lead to denial of problems. In particular, hypomania may cause an overconfidence which leads to an indifference with regards to negative emotion, so that these become repressed. When these become too obvious to still be denied, they at once come to the surface and may thus trigger a depression.
At this point, the hypomania may either gradually turn into a depression, or it may first turn into a mixed state: a nervous state of feverish excitement. The latter happens if the bipolar person still tries to retain his or her elevated level of energy despite the agitation, but this eventually becomes to painful, and he or she finally gives up to end up in a full-blown depression.
On the other hand, depression may then pave the way for a new hypomanic episode in a bipolar person. The withdrawal caused by depression gives one time to contemplate what one wants to change in one's life, so that, when one endeavors to achieve these changes, one's quality of life may improve. At the same time, one may become used to dealing with negative emotions.
Bipolar people often learn to objectivize their emotions because they are so changeable, thus making it clear to them that everything is highly relative. Realizing this, people with bipolar depression may be more likely in a depressive episode to work about their condition than someone with unipolar depression, knowing that a hypomanic episode may come afterwards which may be worthwhile.
Either how, depression may paradoxically improve one's outlook on life in anyone. It puts one before the choice between suicide, suffering or self-improvement. Eventually, if given enough time, one either chooses for suicide or self-improvement. One becomes forced to become more conscious and more constructive in one's thoughts and actions.
Moreover, depression creates an emptiness which forces one to detach from things one formerly took for granted, so that one is reminded of their value, and later becomes more capable of appreciating them and being thankful for their existence. In major depressive episodes, it may be that one has to let go of absolutely everything one ever enjoyed, simply because one has lost every bit of enjoyment of it. But only once you've let go of something can you be mindful of what it means to you. Becoming depressed and then recovering is like dying and then being reborn again.
Bipolar disorder is common among artists not only because artists are often more intense people, but also because art can be a way of dealing with emotions. In addition, however, their emotions may depend strongly on their art, especially because artists often have less social contact then other people. At the same time, their art depends strongly on their emotions. This codependence may spiral in turns into a depressive and a hypomanic episode.