The pursuit of happiness is the engine behind all human behavior, (it is the Positive Drive Principle), but if you look around it's clear that people pursue happiness in many different ways. Where does that individual expression come from? Why does one person see their best path to happiness in heavy substance use while the next person does not? The most basic answer is that all people have free will and mental autonomy, two aspects of human consciousness that allow each person their own unique thoughts and perspectives on the world.
Free will is simply the ability to choose; a uniquely human attribute that arises from the depth of our consciousness. People can store and retrieve knowledge of the past; develop abstract conceptual knowledge; and reason logically with such knowledge; then they can use all of this to project the potential outcomes of several potential courses of action at any given moment. They can choose because they are conscious enough to be aware of more than one potential course of action at any given time (and they are not ruled by instincts like animals). This means that you choose your thoughts, your beliefs, the level of mental effort you will put into your decisions, where to focus your attention, and of course through all of this you ultimately choose your physical behaviors.
Mental Autonomy is another uniquely human attribute, which makes each person the master of their own mind and thus their own behavior. You are conscious of your world through your own senses, and through your own experiences; so whoever you are is made up of this collection of conscious experiences and unique vantage point. Nobody can walk in your mental shoes (or see through your eyes, as it were). This is a realm that is only accessible to the individual who experiences it. Likewise, your mind can't be controlled by other people or circumstances, but rather, you are in control of your own mind at all times. You're the only one in there.