Why is being insensitive a basis for criticism, and being overly sensitive also a basis for criticism?

I see comments on Fluther, and elsewhere, that someone is insensitive, or less frequently that they are too sensitive.

These are apparently strangers telling strangers that their sensitivity levels are just not quite satisfactory and the implication is that this should be a source of guilt, shame or remorse.

So what is the measure of “just right” sensitivity?

And are the same critics of the wrong levels of sensitivity the ones that get to judge the “just right” level?

What power do you think they should have over you if you never quite get it dialed in “just right”?