It's called the "Bond phenomenon" where apparently every Pokemon has exactly one human they're meant to bond with and if they do they achieve a special form of Mega Evolution which psychically links their minds. But since the chances of the Pokemon meeting it's designated human, being caught by its designated human, then reaching the extreme level of trust with it's designated human necessary are so astronomically low it's only been documented a handful of times across history.
It got a cool theme in the Japanese version which is honestly the only reason it's so popular.
https://www.youtube.com/watch?v=SW7QU72Eo2s Also it appears to have been retconned in Journeys.
