I once read a quote that said "I am the combination of everyone I've ever met." To me this quote relates to the question because our experiences, from the worst day of your life to the best, build who we are as a person. They in a way identify us and over time define us. An example would be Chris Mccadless who's experiences in college and 12 years of education before that helped him decide to drastically change his life.
A single decision can change your life forever and not always in a good way. An example would be Amir who lived a fairytale life until he decided to not help his friend in need. Amir eventually goes on to move to America and get married but even after his life seems perfect he stil can't erase what he did to Hassan. Still many many years later the decision he made the day of the kite race effects him.