None. However, part of the role of government (I would likely say it's entire role) is to make society as a whole better. It does this by imposing rules (obligations) on members of that society (taxes, a force to enforce the concept of private property, etc).

In the toy case described above, it is likely to the benefit of society to have both members survive and one member be unhappy; rather than have one member survive, but be happier. Hence, the government enforces an obligation on the first person to share their food.

It's not a straw-man description of Pareto efficiency, it's a toy example designed to answer a very explicit question by the GP. What I described was a Pareto efficient distribution, and not the most welfare enhancing one.

What if neither person can make bread and the other just happened to inherit it all from their parents? Who is burdening who?

And that's before we get into the real-world ethics of whether we can honestly claim that contemporary wealth distributions are really derived from consensual interaction. Why should the descendents of nobility be entitled to wealth deriving from what are effectively violent takings?

I don't know - the whole deontological libertarian project in this sense seems like it has gaping holes.