Unlimited wants and limited resources
Everyone has limited resources whether its time, money, energy or skills that kepp us from maintaining our goals, needs or dreams properly.
It should be clear to all of us that there are insufficient goods to satisfy all of our wants and needs at one time.
The purpose of the economic activity is to provide for us as many of our wants as possible. The shortage leads us to make choices. If we are careful and rational we will chose the thing that is the most beneficial to us.