Veela are sought after by men and wizards alike. It was even worse in the past during the time of emperors and warlords. To protect themselves, veela grouped together in colonies, safety in numbers and all. While this protected them from some of the smaller groups, and even the occasional monster, this made them a target for larger groups like armies.
A solution was proposed, to protect themselves from monsters they would make monsters of their own. They needed them to be loyal and their magic did not lend itself to the creation of golems, so they chose a readily available animal known for its senses and loyalty. Wolves. Their plan was to bond this with a human. Veela themselves could not do it as the animal spirit would clash with the bird they already possessed. Their sons on the other hand would serve quite well as a host. This would also serve as a way to keep their sons close, rare as they were. Before this they would have to send them to a nearby village before puberty, as even they were affected by the allure of so many.
The plan worked and the wolves protected their masters with their lives. An unintended side effect was that the wolves were able to spread the curse to others who survived their bite. The curse would be known as lycanthropy by wizards. As the attacks on veela colonies became less common, the need for guardian wolves became almost nonexistent. Most werewolves nowadays are feral because they have not bonded with a veela who can quell their rage. They are preprogrammed to kill anything that is not a veela or an animal.
Harry is bitten at the end of third year by Remus, maybe by Fenrir who came with Voldemort, or perhaps somewhere in between Halloween and Hogwarts. Fleur shows up for the Triwizard Tournament and Harry feels an odd pull towards her and is strangely protective of her.
Alternative: Would be each veela gets a wolf at a young age, and Harry was Fleur's. This would be more like all the bodyguard Harry fic's with a dash of French Harry.