DocMoriartty said:Yes thanks. A dragon can cast the spell at another dragon. It makes more sense for creatures to know how to charm creatures similar to itself before it can easily charm other types of creatures.
Not unless the creature actually has much knowledge of magical theory and magical biology/psychology, and creates the spell itself. The vast majority of spellcasters get their spells from someone else - the same spells get handed around. Your own knowledge of your species means little if the spell was designed by someone else.

Think of it this way - there are loads of humanoids out there, and most of them aren't very powerful. That means many low-risk opportunities to test out Charm Person spells, until you find one very good and simple one. The number of chances you get to test Charm Dragon spells before you get eaten is pretty small, so nobody's found one quite as good or simple.