I believe that the sense of "moral obligation" to make the world a better place can be traced into instincts. Humanity has relied so heavily on co-operation, that being one of the only ways we are able to survive, that we have this idea to live for the world and not just for ourselves tied into our genetic code. It's similar to how dogs always see things as staying with packs and live for the better of the pack and not just for themselves.
However, this can also be tied down to a more selfish reason. We may be trying to make the world a better place because we are part of that world, perhaps our want to help other people can be again tied down to evolution. Where, when we were cave-men, we had relied on each other to survive, so we protected other people in order to protect ourselves. But most of this is through more sub-conscious means, so we may be morally obligated to help people through these traits, but we may not notice it to such an extent that it actually is.
