Human decision making is extremely flawed. Beyond decisions that are bad in hindsight, we make plenty that we know to be wrong in the actual moment. Procrastination, shyness, fear, tiredness, weak will - these are all examples of failing to do the obvious thing. I call these mental barriers "up or down" moments. When you break through a moment like that, your will improves, you receive feedback from the world and your future becomes a little better. When you fail to break through, you are disappointed, less likely to overcome them in the future resulting in a downward spiral.
A few simple rules and frameworks govern both microscopic and societal phenomena. Science has repeatedly empowered us to do more while a mastery of macroeconomic principles has created fortunes for a select few. Information is now abundant and cheap. Through simplicity, we can distill this information and develop an operational understanding of the world and aspects of it that are relevant for our goals. The key is to understand things only to the degree that they will be useful for. Please note that this is far from claiming that the world is actually simple for the purposes of understanding everything.