'Don't judge a book by its covers' is what people always say, but in this case I'd say 'don't judge someone by their appearance'. 

Sometimes when someone gives off a 'I don't give a shit' vibe, it can mean the person gets hurt easily(not literally, but it's still possible that someone's skin is fragile). 

He/ she might get easily hurt by the actions and words of someone close and that person might never know of it. That just goes to prove that our words and actions are powerful. 

Be careful of what you say or do, but remember: life is not worth living if we're not happy.