Living mindfully is inspired by the principle of opportunity cost in economics. It's a concept that helps in decision making. Everyone everyday every minute, from the various options given, has to choose.
The principle helps to choose the next best alternative. When we choose the other we loose. it is a cost. It pinches because we as humans do not want to loose.
When we choose mindfully, we are satisfied because what we have left will not be our concern. It will no longer be a cost but now what we will focus on will be the benefits of our choice.
So living mindfully means having the opportunity to choose the best option out of the many alternatives.