The document discusses the 90/10 principle, which states that 10% of life is made up of external events outside our control, while 90% is determined by our reactions to those events. It provides an example of how reacting negatively to a spilled cup of coffee can ruin the whole day, while responding positively avoids stress and conflict. The key message is that while we cannot control what happens, we can control how we respond, which significantly impacts our experiences. Applying the 90/10 principle by not letting negative external events affect us can result in a more positive and stress-free life.