Different dynamics exist when it comes to online and offline interactions. The biggest difference is that when a person is online, they do not see a person. More specifically, they do not see the non-verbal types of communication that exist in our world. The roll of the eye, a grin, body language, and the like are completely missed when it comes to online communication. Because of this, there is less sensitivity, as we miss these marks of communication.
In addition, because the idea of another person is not front and center, this lack of sensitivity could cause a person to minimize inhibitions. In other words, what you would never say to a person face to face, you might say online. This is why cyber-bullying is such a big problem these days.