For a dragon, I'd agree that he's more like a hireling. Think of the Dragon Riders of Krynn, or Eragon and Saphira; no matter what else is going on, between them, their relationship as rider and mount is one of friendship, and respect. The same would be true in D&D; unless you captured a red dragon as a small specimen, and tortured the crap out of it, entirely breaking its will, the only way to get it to carry you is to get it to want to carry you. Even the Githyanki only get red dragon mounts because they have an arrangement with Tiamat, herself, and even then, some reds don't deign to serve.
If you were going for a good dragon, you'd need to persuade it to help you, through good role-playing, and if your cause is just, and the reward is worth it (sorry, but even metallic dragons are a bit greedy; they're dragons), it'll serve alongside you, because it understands that you can help it, too (another set of eyes, and another set of hands that can hold powerful weapons, like enchanted bows, or true dragonlances.) if you and the dragon are friends, it might certainly allow you to ride it, when there is need, but if it decides it doesn't like you, it'll leave.
Evil dragons are much the same, even if they don't want to think it. You'll need to placate it, suck up a bit, and make it worth their while, but you can bring the same advantages, and something that powerful will have something it wants; something you might be able to help it get.
As said, though, it's always more of a hireling relationship, or a real friendship, in D&D, just as it would be with a unicorn, nightmare, or other exceptional mount. Even the dumbest white dragon is still a D&D dragon, and they're in the title for a reason. You won't force one to carry you, barring divine intervention, or the most potent mind-control magic, and it will only continue to serve alongside you as it sees fit, but if you can manage, you have one of the most powerful creatures that you can ride, supporting you while you hurl spells, fire off enchanted arrows, or just lead an army into one hell of a battle, from above. To use it again, those in Dragonlance who could ride real dragons, be they good, or evil, were forces to be reckoned with.
I do like the house rules for taming more mundane animals as mounts, even if it wouldn't work on a dragon.