People find careers based on incentives. They settle for jobs based on pay, benefits, or maybe just on what they enjoy the most. For the most part, people don't consider the roll-over benefits that their job will have on other people, only how it will benefit themselves. These seems like a selfish way to choose which job or career path you will take, but it is actually the best thing to do in the interest of the rest of society.
When people have incentive to do something, they do it well. When a job is well done, it creates a positive affect on the rest of the economy. An example of the opposite side to this would be the Communist party and how their jobs are chosen. When jobs are chosen for us or we do not have an incentive to work, we become sloppy in our labor. We don't care as much which results in health hazards, products that are not up to standard, and less motive to find better ways to get a job done.
The benefits of a job well-done flood over into the economy as positive externalities. For example: my dad loves his job as a heavy equipment operator during the summers. Because he loves it so much, he puts extra care in making sure that the roads he is helping build are smooth, without pot-holes or small raises. Those who happen to drive on these roads may not consciously think to ourselves, "Holy moly these here roads are as smooth as babies." But when faced with the decision of driving on that road vs. a bumpy, pot-hole filled road, we will probably choose the smoother one. This is why we should be selfish and go for whatever job we have the most motivation to do. Whether our motivation is money, food, or happiness.
"Choose a job you love, and you will never work another day in your life." -Confucius