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>Lore wise, Mega Evolution can only be achieved through a bond between pokémon and trainer
>Both Mega Evolution and the affection mechanic were introduced in XY with Pokémon-Amie
>Both the anime and Gen 7 dex entries state that mega evolution is actually painful to the pokémon
With all that in mind, here's how I think it should work...
>Mega evolution gives a power boost to the Pokémon but also damages them in every turn (a glorified life orb basically)
>Affection however can reduce the hp drain, with max affection getting rid of it altogether
It's a good thing that Hydreigon doesn't have one. Could you imagine Ghetsis in the eventual Unova remakes with a Mega Hydreigon?