@Umesan that's one of his concerns, it shows up on one of the pages.
Since he's the main character his life probably "isn't worth living if I can't touch my loved one" or he can't bear the fact that he will outlive her(I assume). So, he'd rather risk dying.
It might just turn out that the demon king is dead and this becomes just a pure comedy, no idea, but it doesn't feel like it.