There are two types of mouth guards, hard and soft.
I originally had to have a hard one because my jaw locked up from the pain of a wrongly diagnosed tooth needing a root canal.
It was pretty uncomfortable, but I didn’t have much choice if I ever wanted to open my mouth normally again.
However, when that situation resolved, I was fitted for one of the soft kind and that works really well. Because it was fitted so well over my bottom teeth, I’ve never had a problem with it falling out of my mouth and it works really well to prevent tooth grinding.
Ask the dentist whether your guard will be the hard kind or the soft one. If he’s insisting upon a hard one, he should explain why. Generally, the soft ones accomplish the same thing but are so much easier to deal with.