Question

Why does the dentist's use of the mirror add to the speaker's anxiety??

Answer 1

As the mirrors present the right side of any thing on the left so the speaker was a bit afraid .

Answer 2

As the mirrors present the opposite or mirror image ie. right side of a thing  on the left, the speaker was a bit afraid.

A dentist mouth mirror is concave in design which provide magnification and  upright images.

These mirrors are beneficial and solves  various purposes such as indirect vision by dentist, and many more.