CREDIT: WARNER BROS. CREDIT: WARNER BROS, CREDIT: WARNER BROS, THE DURSLEYS MAKE SENSE NOW... From We know from the Trio's experience with the locket, Ginny's experience with the journal, and the Gaunts with the ring that being around a Horcrux for long periods of time has severe side effects in regard to mood. It takes what is inside and makes it even worse. The Dursley's never wanted to take in Harry, and wanted nothing to do with the Potters. They take him in, and are now around a Horcrux constantly. Feeding him, cleaning him, dressing him. They're constantly exposed, day in and day out. This is going to take their already existing resentment and turn it into near hatred. They lock him in a closet as much as possible (similar to how the Trio put the locket in Hermione's pouch). After even multiple days, their mood lightens enough to unlock the door and once again be exposed to the effects of Horcrux. One last bit of evidence is how the Dursley's seem to become slightly better people, even to the point of Dudley shaking Harry's hand, the less they see Harry as he gets older. They're recovering and healing. - iFunny
