Why is Israel the apple of God's eye?

Up, Zion! Escape, you who dwell with the daughter of Babylon." For thus says the LORD of hosts: "He sent Me after glory, to the nations which plunder you; for he who touches you touches the apple of His eye. For surely I will shake My hand against them, and they shall become spoil for their servants. Then you will know that the LORD of hosts has sent Me. (Zechariah 2: 7-9)