The Brain doesn't want to feel all these emotions. They don't belong to him, anyways. He knows she won't ever learn. He doesn't want to be in her shoes. The responsibilty is forced upon him, and everyone tells him it's an honor to be the one everyone looks to for instruction, but the grass is always greener and the sky is always a brighter shade of cerulean.
"You'll need more protection than that," masculine Brain tries to reason with dainty Heart.
The Heart knows no logic, though; she wanted what she wanted.
"I never knew it would hurt this bad."
"Be more prepared."
She's alone again, because she feels her distant counterpart, Brain, inching away from her. The Heart knows why her more reasonable acquaintance doesn't bother saying "next time." It's because he already has, so many times. It would take shock therapy, or a defibrillator, to teach her about the dangers of feeling and not thinking. This is the reason she sang, though; it was the reason she thumped oh-so erratically while Brain processed information about the new lover's eye color or how he combed his hair a certain way today.
Then again, Heart recalls a low thud replacing her normal rate when everything went down hill. The Brain was always there for her, she remembers. He comforts her at night with words that remind her logic isn't always stoic, unforgiving, and cold. Reason was lenient and understanding to a point.
The currents behind their embankment no longer threaten to flood, returning to benevolent lapping at the shore. Brain notices a calm flutter instead of hysterical beating. He is always taking notes of how to steady his only lifeline. That's his function and so long as they are a duet, he'll be the moon and she'll be the tides.