I'm not sure how something being, "obligatory," makes it something anyone would choose to do, and that is the question. So what if it's obligatory? Why should I choose to do it? [I'm not making an argument here, just emphasizing what the question actually is.]
Put it this way: if a moral imperative did in fact issue directly from God, with a capital 'G' then you would be obliged to obey. Such a directive is, in the jargon of epistemology, indefeasible, and this simply follows from the assumption that god is the final and absolute moral authority, so there is no deferring or second guessing. This is what an absolute is all about. If the the authority did in fact have this grounding, it would be irresistible.
But of course, there is no God doing this. I use the idea to demonstrate the moral grounding you're looking for.
The idea here is that exactly this kind of moral grounding is present in the material, if you will, "presence" of a moral issue
, and the moral authority issues from the value essential to the matter: whether it is torture or just telling a convenient lie, what makes the matter moral at all, is the material ground that is value-in-the-world, that IN the matter at hand, for telling the lie, e.g., puts one in the cross fire of competing obligations, adn when this is given it moral analysis, and what is valuatively arbitrary is dismissed, as this refers to the facts, as Wittgenstein would put it (see his Lecture on Ethics, Tractatus), or, the factual entanglements like you sister's busy schedule that you might obtrude on were you to lie, or a previous promise you will be breaking and so on, all relevant to entangled moral issues, but not relevant to the metaethical/metavalue issue here, and what remains is value qua value, and THIS is the essence of ethics, what makes ethics even an issue at all. No value, no ethics.
Value is not a contingent matter; it is absolute. This is a thesis that can be elaborated on, but it depends on if you are interested enough in the thing you seek: an absolute basis for moral obligation that is grounded in the world and not dogmatically. Let me know.