Many heroes—and lately, some heroines—have a single event that caused emotional damage to them. In historicals, often, a man is rejected by the woman he loves (or thinks he loves), and so he swears off women entirely. In romances across genres, heroes saw their parents with unhappy marriages or domestic abuse, and they have resolved never to fall into that trap themselves.
Some of the past events seem as though the characters should just get over it, already (not every woman in the world is as heartless as your ex, for example), while other events truly seem as if they could do damage.
So what's the most damaging single event you've read in romance novels?