Guide for Living
One important job a philosophy has is to give you guidance in how to live your life. We're constantly confronted with decisions in our day to day lives. Every time we're given a choice, we have to figure out how to make it. Which of the two options should we pick? In philosophy, this topic is called morality. The term is normally associated with doing something tough for a "good cause". While that is a kind of morality, it has a much broader meaning. Every decision you make must be guided by some kind of decision making process, and that process is morality.
Imagine you have a simple choice to make. Say you get invited to two events. One is with people you like, it may benefit your career, and will be a lot of fun. The other is unpleasant, you don't like the people who are going, and you gain nothing from going. Which do you pick? You probably picked the first choice, but do you know why exactly? Obviously you benefit more from it overall. But you're making an assumption. You're assuming that you should do whichever action that benefits you more. There's nothing wrong with that assumption, but the goal of philosophy is to show you how you go about making decisions, and then letting you decide whether your method can use some improvement.
Objectivism says that in order to weigh your various choices, you have to have a way of comparing them and deciding which is better. The way you do that is compare them based on some standard by which you judge the two. For instance, you could judge the two choices by which would make you feel better. Or maybe which would make you the most money. Or maybe whichever way you feel like. There's plenty of ways to compare them, but you have to have some way.
Objectivism goes on to say the proper way to compare them is by asking which of them best promotes your life. By focusing on your life, you can see how the different factors improve or subtract from your overall life. You don't need to only worry about money, for instance. When you understand the positive effect money has on your life, you can weigh it against other things in your life. Romance and friendship can improve your life. A job you like can make you happy and can also give you the motivation to go further and improve yourself. All these things, and many more, have a positive impact on your life. And if you take the time to really think it through, you can try to estimate the relative importance of these things in your life at any point.
Objectivism says you should do what's actually in your rational self-interest. That means you shouldn't just say that it benefits you. You should be able to see how it does. And you shouldn't do things just because you feel like them. Your feelings might not make sense. When people just act on any random feeling, it ends up hurting them after not too long of a time. The consequences come back to haunt you. While you might have enjoyed your actions briefly, the cost is probably too high.
And so that's a strong reason for having this kind of guide to living. You want to be able to see what your options are, and how they'll hurt or benefit you overall. You want to see clearly the consequences, and be able to make an informed choice by weighing those consequences. You have to make choices. But you do get to decide whether to do it in a way that will best benefit your life and lead you to happiness.