That concept encompasses a ton of gray area. For example, did the ancestors of the person you are stealing from enslave your ancestors? Or more simply, are you or your children going to go hungry?
At the limit, the rule is always might makes right. Until then, the question is how much are you willing to give up to re-order the status quo? (A reordering that may or may not end up in your favor)
Almost everyone can be put into a situation where they rationalize a “negative” into a not negative, most easily visible in cases in which the item being disputed is a scarce resource essential to survival.
> the concept of that's not yours and is actually someone else's isn't the most compelling reason?
At its core, this concept is more of a truce where multiple parties agree that the costs of physical violence are not worth it, because the alternative is more acceptable. Hence the saying that “society is only 3 or 6 or 9 meals away from revolution”.