For me it is mask fitting, I involuntarily scrunch my face when I concentrate- no matter what the activity. While diving, this breaks the seal for my regular mask. I can dive with a regular mask- and in fact carry one as well as a spare second stage set up with quick disconnect hose. My buddy also has a quick disconnect hose on his BC. However, when I dive with a regular mask, I am clearing it a lot- so much so that it is hard to really enjoy the dive. I’ve tried a few different styles and brands of masks, but haven’t found one that solves this for me.
My full face mask just makes diving much more enjoyable. I have a Neptune GDivers and spent time training in the pool, and practicing to make sure I can get it on and off, and switch air sources underwater.