Why is it that when one man builds a wall, the next man immediately needs to know what's on the other side?

The line points to a recurring human impulse: boundaries provoke curiosity. A wall is meant to separate, protect, or forbid, yet its very presence creates a mystery—what is being hidden, excluded, or protected? The quote can be read as a comment on power and control (those who build walls define insiders and outsiders) and on the inevitability of transgression (others will test limits, seek knowledge, or challenge authority). In a broader literary sense, it also describes how stories work: a barrier in a narrative invites the reader to imagine what lies beyond and to demand revelation.