The 90/10 principle states that 10% of what happens in life is outside of our control, but 90% is determined by our reactions. How we respond to situations, both positive and negative, largely dictates our experiences. The document provides an example of how one negative event, coffee being spilled, can lead to a terrible day if reacted to poorly with anger and blame, but remain an isolated minor incident if responded to calmly and without overreaction. Mastering the ability to control our reactions allows us to avoid unnecessary stress and problems in life.