The insinuation, therefore, is that if your heart is made of wood, it's very difficult to break.
I'm a little embarrassed to admit it, but I'm a rather emotional guy. And many things break my heart. Most of my career has been spent looking at the terrible things that are wrong with this world. I spent 16 years in television news telling viewers about all the horrible crimes and catastrophes that have damaged and destroyed lives. Now, I work for an organization that's all about doing something about those horrible things. Still, by the very nature that Compassion International is here to help release children from poverty and all of its trappings--child prostitution, slavery, abuse, neglect, hunger, disease--I am still exposed to all of the ills of the world.
I've often wondered why God would constantly and consistently put me in a position where I have to be exposed to such things. Perhaps He knows me too well. Perhaps He knows that if I lose sight of those things, my heart will turn to wood. Nothing will move me. Nothing will break me. Then, what good am I to Him?
There's another great old song that speaks to this, by Petra:
I pray that God keeps my heart from turning to wood. If you're already there, I pray He softens yours. Let something break your heart today.