For instance you're at the bar and want to talk to this girl. You're in the city, a 45 minute train ride from your house. The probability of ever seeing this girl again is close to 0%. You give yourself reasons not to go up and talk to her and then stay content drinking in the corner with your friends. Think about the few times you've done something contrary to what your usual timid self would do. Something like this could easily change your life for the better and you have literally nothing to lose yet you do nothing. This type of stuff happens multiple times a day, your life could exponentially be better daily by going out on a limb from time to time. It's crazy to think you're always limiting your potential and happiness because you worry about people who you'll never see again.