What is the greatest financial lesson you've ever learned?
You're a monkey. It all comes down to that. You are a slightly clever, pants-wearing primate. If you forget that you're nothing more than a monkey who has been fashioned by eons on the plains, being chased by tigers, you shouldn't invest. You have to be aware of how your own psychology effects what you do. This is why we as investors sell at the bottom, get panicked. All the other lessons I've learned have come out of that. As has the field of behavioral economics.
Wall Street clichés, like "cut your losses and let your winners run" come back to prevent the monkey part of your brain from doing what it does. There's a banana--I want it. That's how chimps behave. Us humans react to greed and fear in predictable ways. We are predictably irrational. If you understand that you can take steps to prevent that--we don't own anything in the office that doesn't have a stop-loss on it. In 2008, we watched the market go down 40%. We figured out we're chimps, and don't let the chimp inside us make those chimp-like decisions.
Every good financial decision I've made comes from, "Wait a second, monkey boy, step back, don't do that." Once you realize how your own brain chemistry works against you, it gives you a chance to not panic at the bottom.