Some people can get really serious in the heat of the moment, while discussing a subject that matters to them.. but in the end what makes internet so impersonal is the fact that you can "log off" easily. You don't need to deal with anyone's hurt feelings or bad behaviour if you don't want to, also the amount of informations makes you forget about things quickly. In "real life" is more important to be way less blunt, it's about what you can lose if you show your true colors. On internet you can get away with it.
