Fair warning - just rewatched Bill & Ted, so this is going to be a bit crazy.
Imagine for a moment there exists a world where “awesomeness” is a true force of nature.
Awesome people have the ability to perform awesome magic by channeling their inner awesomeness to their target. This may be used both offensively (a monster might be hit with such a pure wave of awesomeness it becomes a fez. why a fez? because fezzes are cool) and defensively (wouldn't you consider it awesome where the dragon flame that's about to burn you suddenly becomes a river of candy?).
Naturally the more awesome someone is the more awesome their magic power is. Almost everyone has a little bit awesome in them, but not dave (lowercase d because he's so boring that even his name lacks any strength of character). dave is so boring and plain that he sucks the awesome out of everything and everyone near him just by being there to the level no awesome magic can happen near him.
Why would a group of awesome magicians keep dave near them during their D&D-style adventures? Not just sorta be OK with him making their magic fail but actively want him around? It can't be that he's cool and fun to be around because that would mean he has some awesomeness in him, which is clearly false.