According to the website Foodbeast.com, the reason that McDonald's fries are addictive is because they're coated in BEEF FLAVORING. That makes them activate your "umami" taste buds along with your salt taste buds, which hooks you.
By the way, that also means the fries aren't vegan-friendly. Not because that beef flavor comes from beef- that would make too much sense. It's because the beef flavor comes partially from milk.