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 bad reaction to a spilled cup of coffee can negatively impact an entire day, while choosing to react calmly would lead to a better outcome without stress or conflict. It encourages applying this principle by letting negative comments or unexpected events roll off without internally reacting in a way that ruins one's mood or day. Controlling reactions is seen as providing a sense of empowerment over life experiences.