From Schelling's The Strategy of Conflict (Section II-4):

[A] focal point for agreement often owes its focal character to the fact that small concessions would be impossible, that small encroachments would lead to more and larger ones. One draws a line at some conspicuous boundary or rests his case on some conspicuous principle that is supported mainly by the rhetorical question, "If not here, where?" The more it is clear that concession is collapse, the more convincing the focal point is. The same point is illustrated in the game that we play against ourselves when we try to give up cigarettes or liquor. "Just one little drink," is a notoriously unstable compromise offer; and more people give up cigarettes altogether than manage to reach a stable compromise at a small daily quota. Once the virgin principle is gone, there is no confidence in any resting point, and expectations converge on complete collapse. The very recognition of this keeps attention focused on the point of complete abstinence.