I'm going to borrow many ideas from birds to keep my dragon airborne, porous bones and feathers to keep it light while spewing balls of flame to create warm air currents at high altitudes. The problem is that during mating season the males will have to fly in a tight circle around each other at a very low altitude constantly burping fire hoping to attract the female. The fertile females quickly shed all the feathers and lie with their back against the ground and growl. Once a male has been enticed by a female it will swoop down and carry the female and perform one of the dangerous mating ritual the world has ever seen; copulation whilst handicapped in the sky. (I shall not describe the ritual in details but it's beyond even your wildest imagination! Just kidding...)
The females, by now completely bare, take a dip into a swamp infested with "giant hook worms" or badly mutated leeches with claws, the ideas is to glue the couple tightly while they engage in a heated exercises mid air. Bite wounds quickly heal and the secretion, which is actually a kind of mild neuro toxic, prevents blood clots and also works the heart muscles. The increased blood flow allows the laden dragon to stay in the air longer. However, there's a mystery that still puzzles researchers (especially me and probably Darwin too) The mystery is why did the dragons choose to perform sexual intercourse at heights? I mean come on with the extra weight and fire thingy evolution can get too excited in a deadly way. By the way I already named this mating ritual, it's called Red Summer Rain!