[@ClocktowerEchos]
1. Because dragons are big and strong, ergo fusing with a dragon makes you big and strong too, meaning you get to eat more for various reasons. I do like your explanation of the idea's origin, however.
2. Energy needs, I'd assume. That is how they are considered in canon, as beasts that "feed on" lightning. I could change it to something else if need be, though... lava and magma are extremely hot, after all.
3. Again a reference to dragon ogres in canon, who are ripped as all hell.