I don't believe they have this constant flame going on inside of them. First of all, lack of oxygen; second, dragons would have to eat constantly to provide enough calories to fuel that fire, let alone their other bodily functions; lastly, dragons too are made mostly out of water and when water reaches a certain temperature, it becomes steam. The only reason I can think of why they don't explode when touching lava is because of extreme outside protection.
yes i know. however while cartoon is a lame excuse WHEN said cartoon don't follow the cartoon physics (Pinkie don't count), MAGIC can be used to make good explanations.
For example: "Dragons are beings that embody most, if not all the common reptilian characteristics. they are also extremely magical beings, one of the effects of their enormous magic is their fire. since gems are often charged with lots of magic, dragons eat gems to fuel their inner fire, converting the energies of the gems, crystals and ores into fire, said conversion is made by *MAGIC*. keeping their inner fire on all the times is not a very tiresome process for a dragon that indulges in gem eating. different kinds of gemstones are harder or easier to transform into fire energy. Fire rubies are a delicacy among dragons for being so easily digested, while water sapphires are very hard to digest. however most dragons prefer to keep their inner fire out. Dragons usually lit their fire when they want to comfort another being. since dragons LOVE warm places, like lava, such action is, pun intended, heart warming for dragons. the warmer they are the more heart warming it becomes."
What do you think?
Also: Spike have free access to lots of gems==> Lots of energy ==> very warm blood ==> Ultimate heartwarming moment with Jade.
"Dragons are beings that embody most, if not all the common reptilian characteristics."
This... is actually not true. They move more like felines and, unlike reptiles, are capable of feeling complex emotions like love. Their legs seem to be positioned beneath the body and they don't seem to be affected by cold temperatures.
I personally believe that while dragons can sustain themselves in extreme temperatures, their interior's temperature must be within a certain margin.