In an emotional sense, hearts are made of glass. So fragile, beautiful, and it can be given to anyone, anywhere, at any time. Unfortunately though they are also so easily broken. Then once it breaks you can super glue it back together, sure. It might even still function the same as before, but it will forever be different after it is broken. When glass breaks, you will always be able to see the crack, no matter how hard you try and put it back together.
Each shard of glass a memory, a thought of them. Some of the small slivers might have gone missing, never to be seen again, or swept into the trash for proper disposal, but the entirety of the glass piece remains. After the first time it breaks you can either hide it from the world and keep it from ever breaking again, or you can continue to show it off proudly and ignore the imperfection. You already know how to deal with broken glass anyway, right? So what’s the harm in another crack or two? There’s enough super glue in this world to put it back together again.