A job or career can be a source of fulfillment for some people. Especially those who work alot, you end up spending more time with the people at work then you do with your own family. So the office starts to feel like a home, the coworkers feel like family, and the work you all do is fulfilling, so if someone is lucky to have all three, they fall in love with their work, and its no longer seen as "work".
It is a life-style, the merging of the individual and (creative) works.
Great achievements require sacrifice, and we try to give meaning to these extra-efforts. In the economic world, this attitude is connected with greed, which is harmful to human peace as vital resources are being monopolized. In the world of creative works (science being an ambivalent activity),which contribute to human progress, we would have no Picasso, Shakespeare, Chopin, etc,—-also no Newton, Einstein, D.Boehm , R.Feymann. To sum it up: human progress requires sacrifice, which can be moderated by creative ability.