As someone who has experienced several manic episodes and one very long depressive episode, I have taken a keen interest in understanding what I believe is the root cause of the episodes. That is, the breakdown of the body's emotional regulatory system.
First you have to understand what emotions are and what their purpose is. The simple explanation is that they are the brain's "muscle memory". They serve as a short cut to determining how you should react to immediate circumstances. You don't have to ponder every change in your environment. Certain changes trigger chemical releases that make you feel a certain way physically. This physical reaction affects your judgment and behavior, especially if it's a circumstance that you've experienced before.
The amygdala is the part of the brain that, among other things, helps regulate emotions, their duration and their intensity. It does so by signaling other parts of the brain to signal certain organs to release specific chemicals. It's not necessary to understand the details of how all this occurs, only to understand that it is a semi-automatic system and basing your judgment and behavior solely on it is not always in your best interest.
I believe, in the case of bipolar episodes, that for some reason this system goes awry. For example, in a manic episode, I was constantly immersed in a feeling of euphoria laced with adrenaline. Every idea I had was brilliant and I wanted to act upon it immediately. If the system were working properly, a great idea or epiphany might trigger this response for a few moments. Instead, it was constant. Whatever is responsible for turning it off was not working. The interesting thing is that it only seemed to malfunction for a particular emotional state. This was glaringly evident in the depressive episode, which lasted an entire year. My brain seemed only to be able to focus and dwell on the negative. Everything was hopeless and pointless. When it finally stopped, there had been no significant change in my circumstances, only my emotional state. It just stopped.
The realization that overwhelming emotion is a sign of a physical malfunction has helped me to be much more aware of my emotional state and to always question its validity. It's been about 7 years since my last episode and I think part of the credit goes to the fact that I keep my emotions on a short leash these days. For example, I recently was feeling very agitated and short-tempered. This, by itself, is not necessarily a red flag. However, after a couple of hours I began to do some self-analysis. There really was no reason for it and it wasn't going away. In addition, I realized that even if there were a reason for it, it's not a very productive state of mind anyway. While I couldn't immediately shut down the physical feeling by shear power of will, I could adjust my thought process and reaction to the feeling. I essentially regarded it as a temporary ailment, like having a cold. I was careful not to take it out on the people I came in contact with and to disassociate the physical feeling from my reasoning process. Eventually, it subsided.
This is important because the brain can be trained. Your reaction to an emotion this time, influences the chemical releases that will occur next time. Your brain stores information on how you reacted and whether the result was good or bad. You may not have to work so hard to control it in the future. On the other hand, if you allow yourself to wallow in an emotion without question, you are reinforcing the chemical reaction which may make it more difficult to counteract in the future.
Like most tools, emotions can be very useful. However, if you don't take the time to learn how to control them and recognize when they're not working properly, they can be dangerous and even deadly. Drugs can mask the problem, and if you need them, take them, but don't be afraid to confront, challenge, get to know and understand the various physical states we call emotion. Make them work for you rather than govern you.