I think because my mood obviously cycles and the episodes are severe enough to interfere in my life. When I get depressed it comes on suddenly and I become suicidal, then it goes away just as randomly. The only thing I've noticed that can sometimes trigger such episodes is extreme anxiety or getting sick. Otherwise there is no explanation. Considering the length and number of episodes I almost fit more into cyclothymia, but the depression is too intense for this diagnosis I guess. Then there is the racing thoughts and sleep disturbance, though I know other creative people without bipolar who can experience this as well...

When I was in my Phd program it was easy to write everything off as stress, but now it's been a year and nothing has changed.