Not all hearts break at the end of a fist. Not all wounds bleed. Some hearts erode a bit at a time. Skin grows calluses with each insult. These hearts grow thin. These hearts weaken a bit with each minor insult, each microaggression or microinsensitivity, each unhappy parting, each conversation postponed and postponed again. Then they crumble at the smallest-seeming jolt. A bump in the road, a promise blown off, a disappointment not otherwise worth noting, and all is lost.
Hide a heart in a futile attempt to shelter it from harm, and it grows brittle. It ossifies. The calcification grows worse with time and lack of use. Eventually all hope is lost: the heart is too shot through with bony crystal. It will never throb with longing, will never beat faster at the sight of a beloved one returning.
Some hearts, broken early, are agaves; tough skin, fierce spines. These hearts open only to reveal that the protected layers inside have spines of their own, each layer in turn bearing scars from the spines that once protected it.
A healthy heart, exposed to air and sunlight and worn without armor, is the worst of all. It will break repeatedly then rage back strong to break again, its owner fated to suffer beauty without end, a Promethean cycle for those who would grace the world with fire.