How can you allow a comment to hurt you? Easy, by being human and having feelings. Comments can't hurt if you're an empty husk of a person who has no feelings; if you're hypervigilant about bracing for attacks; or if you never take a risk of being vulnerable and never share anything important about yourself. None of these options is particularly healthy. Having no feelings is a type of major depression, and living in fight-or-flight mode will lead you there, or to an early grave. The last option is at least reasonable online (but not in relationships), but not so easy in practice.
A common theme in the responses here is the element of surprise, comments and criticism that blindsided the person by hitting them in a vulnerable spot that they didn't know that they were exposing.
That certainly comports with my experience in receiving hurtful comments.