They obviously are intelligent, they have their own language, so as intellectual beings you should be able to befriend them somehow, maybe you could approach a lair cautiously, put down some special items gems, gold, meat, or flowers and creep away. and eventually (and several thousand in gifts) you could approach one. this would make it a trade-off. Either defeat it and absorb its power or chose to befriend it. The friendship would be precarious at first, snapping at you when you get too close or startle it but the rewards eventually would be greater than svcking its soul (or whatever thats called). The benefits would include air combat, extremely fast travel, fame (I mean if you ride a dragon you've got to be pretty awesome), gifts from it, and just the plain joy of riding a freaking dragon. Also, befriending the dragon to a certain degree could have it grant you special powers, similar to those you would gain from defeating it. Another, quicker, cheaper, approach would be to defeat it but hold the final blow, starting the relationship. Or a dangerous approach, steal an egg, and wait for it to hatch. This could lead to problems later on though, specifically when the mother finds you. depending on how intelligent Bethesda chooses to portray dragons, this could test the dragons loyalty, or just result in a battle with a very angry mother. there could be other, more unique ways of aquiring dragons, such as having one being left on your doorstep by a dragon for some reason or another.