As I've observed sarcasm in social interactions, I've noted that those who use it tend to underestimate its negative effects because they assume that what they say is humorous instead of hurtful. People who use sarcasm often think their targets are too sensitive or naïve when feelings get hurt. “She just can’t take a joke,” they say. In more disturbing cases, sarcasm communicates contempt for others and gives people the “dishonest opportunity to wound without looking like they’re wounding.”8 If someone feels hurt by such sarcasm, the one who made the verbal jab will often respond with something like, “I was only teasing! Lighten up.” ~Jennifer Grace Jones, Ensign. Aug, 2013.Sarcasm, as I understand it, is irony directed at a person. It's saying something we don't mean. Which, by definition, is not truthful. Now, just to clarify, I believe there is a difference between sarcasm directed at a person and being facetious about a situation. Facetiousness is saying "Lovely weather we're having" while you and your friend are running through pouring rain. (Unless, of course, you happen to adore downpours. In which case that's simply a statement of opinion.) But when sarcasm is directed at people, it can have a wounding effect. Even though we don't *really* mean what we say.
Think of Sherlock, or House. Hilarious on television, but would you really want one of them in your circle of friends? They make people cry on a regular basis, and we laugh. However, in real life, that's not so funny. Sarcastic humor can be contagious. It's a habit that I've sometimes fallen into, and it's not always easy to break.