Being more assertive can lead to a more rewarding life. It can improve upon existing aspects of your life (such as better relationships with friends and family) and completely change or introduce other aspects (such as personal or career opportunities.)
While the decision to develop your assertiveness is a personal one, the rewards can be much greater than the personal satisfaction that being assertive brings. Being assertive not only improves your self-respect, but also earns you respect from those around you. The change in your behavior can positively influence those around you, both in their behavior toward you and also in their behavior toward others. For example:
You and your friend go to the store to return a piece of broken merchandise. After being turned away by the clerk, you ask to speak to the manager and convince her that the store should return your money. The most tangible reward of your assertive behavior is that you get your money back. At the same time, you feel good about standing up for your rights, and your friend is impressed with the way you handled the situation. The next day, you feel more confident at work and suggest an idea for a new project. Your boss likes the suggestion and asks you to oversee it. Meanwhile, your friend, who doesn't understand a bill she's received, remembers how you handled the situation in the store yesterday and calls the billing company for an explanation. She feels better about herself and more confident about speaking up the next day at work, and so on ...
While returning one item to a store may not get you a big promotion or drastically change the lives of those around you, the combined effects of multiple acts of assertiveness will increase your confidence in multiple areas of your life. And the more positive and fulfilling interactions you have with friends, family, and coworkers, the more these areas of your life will improve.