Who keeps you on your toes?
Be it at work, or at home, you always have some responsibilities on you. It is only natural to do just about enough to either meet or slightly exceed the expectations of someone who is also concerned about these responsibilities. This someone could be your boss, your spouse, your parents, or even yourself. The effort you put into fulfilling the responsibilities will, for most part of the time, be determined by the expectations of this person (or people).
If this person is pushing you to your limits, not by force or intimidation, but by raising the right questions that make you feel you can probably do better to answer them, and is motivating you to do better, your work and your actions will only improve with time.
If this person fails to constantly keep you on your toes and maintain your drive, by setting the bar of expectations too low, you will fall into a pit of lethargy and will have to find another person to bring out the best in you.
This is true of brands as well. If a successful brand is not kept on its toes, by constantly changing and constantly improving, a competing brand is bound to overshadow it in time.
It is important to choose that person who can bring out the best in you (doesn't matter if you're a person or a product). It can be someone in the morning and someone else at night. It can be you all day everyday. And it is just as important to switch from one person to another when things aren't going as good. The hardest part is to tell when things aren't going as good.