We all do it. But sometimes that kind of backfires on us doesn't it?
What I mean is maybe someone is around listening to what you have to say, or you're probably just cracking jokes with someone who you THINK is your friend? A phrase that I like is "Be careful what you say; the walls have ears." which is not literally true but is a great metaphor, just be careful about what you speak and who you speak around. But has this ever happened to you? Where you think "OH they'll never find out about what I said" but somehow they do? Sometimes you feel guilty or regret what you've said.
Other people criticize, because its a way for them to feel more superior than others; By kicking someone who's already down. But what we don't realize is people who do that stoop to a LOW level. That's their way to feel better than everyone else. I can't lie I've criticized many times and have BEEN criticized, i've never tried to criticize with bad intentions or thought that anyone tried criticizing me with bad intentions. But it has affected me later on. Maybe the person you were gossiping with a year ago now does not talk to you and just to weigh you down they tell that person as if YOU were the only one doing it. Things are often misinterpreted and become a bigger issue than they already were. Sometimes they aren't even issues to begin with. My point is some teens even adults refuge themselves with criticism/criticizing but we should think of how that person would FEEL if you were to say all those negative things to THEM. Basically you're being a hypocrite.
P.S. Whenever an issue occurs because of criticizing don't jump to conclusions and attack them back. That'll only make the problem worse.
Remember, COMMUNICATION IS KEY.