Sustainability

It stems from two things: survival and respect.
We have one earth: finite resources. The only thing continually entering our system is energy from the sun, and even that will run out someday (fortunately not on human timescale). Quite clearly, we, as a species, desire to continue to exist. In fact, life in general has an overall tendency to continue to be, an inclination towards survival, a habit of keeping the earth's surface humming with the vibrations of organic existence.
Humans have, unknowingly at least to the majority of the population, overused resources contained by the earth, or we're at least on path to overuse them. And perhaps more significantly, this overuse has dramatic effects on the environment, which threaten all life. Undoubtedly, humans will find great challenges to their survival with a severe climate change. At this point, many people are ready to announce a future of great suffering, but I hesitate: perhaps I am too young to be so pessimistic.
So sustainability is about shifting to a way of like which will sustain humans for another 10,000 years. It's about survival. In this way, no-one can have any legitimate argument against sustainability. It's in their best interest and the best interest of their children. But many people perceive sustainable development as something which will interfere with their level of living. They'll have to give up something. Well, first of all, maybe they should be giving it up anyway, but for the most part this simply isn't the case.
Sustainability isn't simply about survival though, for it was, it could be rather brutal. It integrates concern over the well-being and happiness of all living beings. It's about respect. And while it emphasizes (admittedly anthropocentric) human interests, it recognizes that human health and well-being is directly connected to that of the eco-system we live in. And in understanding today's society, and the role of the economy, it recognizes that any effort to integrate sustainability must fit, at least in some ways, with the economic system. Hopefully, it will gradually change the value system inherent in the economy (i.e. away from the idea that money is god), but at this point, it will work with the system to accomplish its goals.
The sources: survival and respect.
The results: environmental protection, social equality, and economic efficiency.

"Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs."
Brundtland Commission - 1987