To defeat the enemy, we must understand them first. There are two fundamental reasons why a troll trolls:

They’re bored: Trolls lack stimulation “IRL” (in real life), for good or ill, so they seek it online where it’s readily available and easily acquired. A troll’s behavior reflects a deep insecurity so having someone respond to their words gives life meaning, regardless of how pathetic that may sound. I raided that wedding because I wanted to be noticed and talked about. Random people cursing me out through private messages or the general chatroom invigorated me. I was so bored with my real life, and even my virtual character’s life, that I learned to find joy in harming others. If a troll had something better to do, like work or a hobby, they wouldn’t have time to troll. The next time you find yourself posting a negative comment think about why you’re doing it.
They want attention: All a troll wants is you to turn the spotlight onto them. They want you to repost their comment to your followers. They want you to write a blog post or status about them. They will use anything and everything to get it. They will criticize you, post inflammatory comments, or write remarks just to make you wonder how someone could be so dumb. The problem is that you will feel compelled to respond to “set things right.” Even if you respond in a cheerful or positive way, you’re still feeding the troll.