"My theory comes from the work done on creating split personalities," Harry explained. "Extreme forms of pain or trauma were often used to cause the splits, to create the alternate personalities. Emotional shock or trauma affects our magic, too. It may have been that Ron used the death of your parents to cause a fracture in your magic, one he was able to capture and exploit before it healed. The constant emotional pain you feel over their deaths would have fuelled any possible enchantment, even if it was just a low-level control one.