Wizards are brought before the dragons and bonded to them under their thrall. They are not pets in the literal sense as they have their own home and lives but if their dragon calls upon them they must obey. The dragons use the wizards to keep humans out of their affairs, ensure their food supply is maintained and other things depending on the dragon in question. How can the dragons keep their wizards obedient and keep this system in check? The way I have it written is, both dragons and wizards are creatures of magic, wizards are brought before the dragons who shout at them kinda like the Thu'um in Skyrim and as the superior more dominant creature of magic this binds the wizards to them. Of course they'd need to have trained, unbonded wizards available for thralling when an older wizard dies or when a new dragon is born. The PR spin would be that in this world dragons are revered and that serving them is a great honour.