The expression "heart of stone" is most commonly used to indicate someone who is emotionally cold, or devoid of warmth and feeling.
I think sometimes that I have one, not by the above definition, but because I've created it to shield myself from getting hurt. Most of the time, I just feel numb.
My uncle chewed on my ass the other day about my negativity (among other things, and for the sake of goodwill, I just really don't want to discuss it), and pulled the equivalent of telling me to a) smile and b) cheer up, positivity breeds happiness! I'm in therapy for depression, and telling someone to smile is a form of bullying. The road to Hell is paved with good intentions, folks. I would love to be all icky-happy, but really, um, no, not right this second, on command. I'm not a circus animal.
I have spent a lifetime being told what I am not allowed to be or feel, and it sucks that others think they have a right to dictate terms about it. It would be really convenient if we could just turn off our feelings altogether so that we never get hurt. A lot of the time, the pain is the only thing I can feel.