Via Firda comes "Why a broken heart hurts so much": “A rejected lover’s broken heart may cause as much distress in a pain center of the brain as an actual physical injury”. That sounds about right, except that it would be hard for me to cite any real experience in the matter. My ex-girlfriend broke up with me—actually, you know, I can't point to a single event where we decided we were broken up. She end up telling me why, though, and it was because I didn't call her enough. There's a perfectly good reason why: “whenever I make a phone call, it always feels like I'm interrupting something.” It's been a rare phone conversation that I've initiated that's gone past 30 minutes. When girls call me, I can and have very often stayed on the line for hours. But we're off topic already.
I can say that I've been rejected straight out twice since then, and each time it had been girls in whom I had invested quite a lot emotionally (read: pined over for a an extended period), and when it came time to profess my adoration, the first thought of me as Just a Friend and the second one wasn't looking for a relationship with anybody at the time. (Rejecting me, that I understood; rejecting any type of romantic relationship with anybody, that took some time to process.) Since then I haven't really bothered. The times I've told a girl that I liked her in "that" way either slipped out or was told in a such an oblique way that she had no idea what the hell I was talking about.
My heart never was truly broken, never truly ripped out of my rib cage, hurled across a treehouse, hitting a wall and sliding into a wastebasket. But it may have atrophied due to non-use, and that could be a lot worse.
