I agree with pragmatist in that someone who has an advantage "should" help others that do not, in certain cases, but I don't think the language of obligations is the right one for this "should". It is more suitably discussed in the framework of virtue ethics. Part of being "a good person" is helping others who ask this kind of favors of you, within reasonable limits. Refusing consistently to do them is not accurately described as neglecting an obligation, if there haven't been any promises/contracts, but it is (to use pragmatist's words) "being a dick"--a character trait it is better not to have.

I don't understand the question. What's a moral mechanism? If you are asking for a general moral principle that establishes the obligation, I should point out that I'm a moral particularist. I don't think that appeal to general principles is essential for sound moral reasoning, and I don't think there are many true, simple and general moral principles.