Sommy's failure to create a mega stone for all Pokemon lead to the accidental development of the awakening stone. Only Pokemon with max IVs could use it's power. Adrenaline constantly runs through the users body from exposure and causes massive mood swings during it. Prolong use can cause permanent damage to the body.
Kecleons often show their mood through their colors and despite Lee getting good at controlling that, the stone made him a rainbow of emotions during the use.