For many people, the characterising feature of trolling is the perception of intent to disrupt a community in some way. Inflammatory, sarcastic, disruptive or humorous content is posted, meant to draw other users into engaging the troll in a fruitless confrontation. The greater the reaction from the community the more likely the user is to troll again, as the person develops beliefs that certain actions achieve his/her goal to cause chaos. This gives rise to the often repeated protocol in Internet culture: "Do not feed the trolls."
Often, a person will post a sincere message about which he is emotionally sensitive. Skillful trolls know that an easy way to upset the person is to falsely claim that they are a "troll." In forums where most users are similar to each other, outsiders may be perceived as trolls simply because they do not fit into the social norms of that group. It can sometimes be difficult to distinguish between a user who merely has different values, views, or ideas, and a user who is intentionally trolling. This can lead to genuinely hostile behavior, including flame wars.

source: http://en.wikipedia.org/wiki/Internet_troll

why would i do something for your forum if you assume i was a troll