In Real Life, this is called tactile defensiveness and is common in abuse victims and people with conditions like ADHD or autism spectrum disorders, and especially with OCD or if the person is Terrified of Germs, but even then, some people just don't like it and can't explain why. At other times, they may already have a special person whose touch doesn't trigger any adverse reaction, indicating a particularly trusting relationship. Most of the time, they succeed but the number of blunders and the time it takes depends greatly on the genre and nature of the story. If it's treated like a problem, expect their Love Interest to attempt to fix it at some point. Sometimes they change for the better later on, sometimes they don't, and sometimes the fact that they don't like to be touched isn't treated as a problem in the first place. There's a good chance that they may be something of a loner as well. Sometimes there's something wrong with them in a dangerous way and they know it. They may be this way as a result of a bad experience, simple shyness, or because they just don't have the tolerance/patience for such things. This is a character who just doesn't like being touched, whether it's by a stranger or someone they know.
