The Dragonlady said that Dawn's pokemon must completely open up their heart to her before they can learn Draco Meteor.

Dawn, having a bit of a fetish for pokemon, decides that the best way to do that is to seduce her young Gabite/Garchomp.

Bonus for hemipenis. Double bonus if even after all the fucking, the Dragonlady still won't teach it Draco Meteor.