One who makes himself cruel for the sake of Torah, or Torah study... isn't doing the right thing, which of course should be obvious, but the fact that self-sacrifice for the sake of Torah can go wrong is a worthy point. Now, back to Eruvin topics - what happens when you have an apparent public domain passageway through what would otherwise be a private domain? Do you have to divert the path? Note that the topography of a given area can make a difference, how you block off private domains. Note that the "physicality" of an eruv is somehow acknowledged as less than the actual lay of the land, or physical boards, and so on.