Think about it. Think about a passage from a Victor Hugo play, perhaps mangled by me but true to the original French:
...Happiness is serious
And asks for hearts of bronze
On which to engrave itself.
Pleasure alarms it by throwing flowers to it.
It's smile is nearer tears than mirth.
Think about the happiest moments of your life--your wedding, the birth of a child, the sight of a parent with dreams fulfilled, or a son or daughter on the way to fulfillment at a graduation or showing off a grandchild.
Hearts of gold are not enough--too shiny and shallow, like party favors. The metal is too soft.
It takes a heart of bronze.