How much are you like your father or your mother? While our experiences and our situations have tremendous impacts on who we are, how we act, etc., our genetic makeup is also important in determining these factors. Therefore, our heritability, or the extent to which differences in a trait can be attributed to our genetic makeup is important in trying to understand human behavior.
Also, keep in mind that genes and environment do not occur in isolation, but interact with each other. As a result, you are who you are, and you act and think the ways you do because of the combination of your heritability and your environment.
Related: Heritability Estimates