>>58034137
I mean, it should have been obvious from the way they were depicted in the anime - the Pokemon in question generally looked to be in extreme pain and had a tendency to go berserk, like Lucario. The whole reason they originally planned for you to need to have high affection to use it was because your Pokemon needed to have the utmost faith and loyalty to you to willingly put themselves through the pain needed for the transformation; the physical strain on their bodies is also why the transformation is temporary, as staying transformed for too long would presumably kill them.