The 90/10 principle states that 10% of life is made up of external events outside of our control, while 90% is determined by our internal reactions to what happens. How we react to situations, both positive and negative, has a significant impact on our experiences and outcomes. The document provides an example of how having a calm versus angry reaction to a spilled cup of coffee can change one's whole day and relationships. It encourages applying the principle by letting negative comments or delays roll off instead of internally stressing over things that cannot be controlled. Mastering one's reactions is key to avoiding unnecessary suffering and achieving a more positive life experience.