#elemental doc

I was talking about this is @jackiewinters years ago lol

As for my own theory:

Doc wanted to spend some time with the inhabitants of Slugterra and stay down-to-earth because he saw that his brothers and sisters (the elementals) were getting more and more egotistical, and to stop that, he transformed himself into a Boon Doc.

This transformation cost him his power level as well as some memories of being an Elemental. Hence why he was getting more powerful over the course of the series until Return of the Elementals where he gained all power and memories back.