Heart creates enough energy to drive a truck 20 miles - or to the moon and back in one lifetime. So when you tell someone you love them to the moon and back, you're telling them you love them with all the blood your heart pumps your whole life.
The Unabridged Journals of Sylvia Plath: "Yes, I was infatuated with you: I am still. No one has ever heightened such a keen capacity of physical sensation in me. I cut you out because I couldn't stand being a passing fancy.