It's also a tactic to use when bullied, but you need to be aware that this is what is happening and take other steps to stop it. Bullies will use any emotion you display against you. Happy about something? It's dumb and so are you. Sad about something? You're pathetic. Becoming an expressionless rock is a viable way to cut off that particular avenue of abuse, although they will likely find others.

If you find yourself doing this out of self-defence with someone, they're likely a bully and you should do your best to cut them out of your life. Rocks can still be pissed on. Another thing to be wary of is that, if you spend too much time grey rocking, you might become a rock to people who are kind to you.

Bottom line, if you need to "grey rock", you have a problem that might require a better solution.