Some people can be the same person face to face as they are on the internet unlike both of you. Try to be "sarcastic" or a "hooligan" in person and you'll have serious issues with the wrong person, on the internet you can be tough to whoever you want with minimal chance of backlash.
I don't act like a clown on the forums because that's not how I act in person. Some people can take the harassment and some people can't. I don't get offended easily so I honestly couldn't care less what you say to me, but when you keep poking and prodding some guy who's clearly flustered, it's time to call it quits. I can guarantee you wouldn't be spewing your garbage trolling BS in a face to face situation, because that's not how normal people interact with each other.