Becoming a parent.
I got pregnant with my son at 18 fresh out of high school. Prior to that I had zero cares besides my friends, boys, and money. I can honestly say my mind set changed as soon as it set in that another person depended on me.
I moved out of my mom's house 4 months after my son was born and have been doing what I had to do to provide ever since.
It was a major reality check.