In "Mirror" by Sylvia Plath, why would some people consider the mirror to be cruel?

's poem,
"," is narrated by the eponymous mirror. In the fourth line of the first stanza, the
mirror says, "I am not cruel, only truthful." The mirror might be perceived as cruel
by some people because it exposes people to the reality of the ageing process.

In the second stanza, for example, the mirror says of a woman who comes and goes
to...