From top to bottom, notice that 7 is between 4 and 8, as the structure follows the normal properties of a BST. We can express that idea with this conditional: if we encounter a node between node1 and node2, it means that it is a common ancestor of node1 and node2.

It also spawns these two ideas: if we encounter a node greater than both the nodes, we're going to find our lowest common ancestor on its left side. Likewise, if we encounter a node smaller than both the nodes, we're going to find our lowest common ancestor on its right side.

Why is that? Let's see the tree expanded a bit.

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/*

      7

     / \

    4   8

   / \

  1  5

*/