How can I go so long feeling completely happy, then suddenly my depression knocks that all down?

Depression is usually a chronic illness. If they are correct, then it is a chemical imbalance. There's no real way to "fix" a chemical imbalance. Medications can help manage it but the hormones in your body are always changing and cycling. So therefore, if you think about it in those terms, you can understand that it's usually not sudden, that you can track small changes and learn to anticipate it. The rest is about learning coping skills to ride it out, keep it from escalating. Understanding your triggers, especially. This all comes with time, practice, therapy and many many ups and downs.

It could be situational depression, or it could be a chemical imbalance in your brain. You should talk to someone about it, and if need be, have a medication evaluation to determine if it is medication related.

Little things can cause things to go bad but it depends if you let it get you down. Think about the good things and do your best to be positive.

Depression can come into your life randomly; It's like a cold, It is inevitable that it will happen.

Because the things that make you happy keep your attention. But after all if you have some psychological problems your mind keeps repeating them over and over again.

Biology can play a large part in feelings such as going from happy to depressed. We all have these natural hormones in our bodies that change levels at different times because of different things (i.e. changes in the weather, our age, the death of a loved one, a new birth etc.). Hormone levels in the body, I believe, naturally usually have cycles. I have yet to meet a human being who is 100% happy, 100% of the time. However, there is a difference between being a "normal/standard" version (on a graph of a bell-shaped curve, you would be the bump or bell part of the curve) of sad and clinically diagnosed depression. Depression can be debilitating (I know; I have it). Sometimes - not always, depression is caused by an imbalance in hormones. Depression can also be caused by other things too. Have you thought about talking with a trusted medical professional about depression, and what you think might be causing you to feel so down? What things or people help you feel better/healthier when you are down? I hope this helps and you feel happier soon.

This could be an indication of bipolar disorder. It is a completely normal thing to feel, sometimes depression can take over our lives but we have to strive to stop it.