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

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

为什么当一个人建了一堵墙，另一个人马上就需要知道另一边是什么?

-- George R.R. Martin 乔治R.R.马丁