Why do Trolls Troll?

I have just seen this post from QAT in the forum:

"*** Edited by QAT ***

This profile is fake. All forum posts made, as constructive as they were or were not have been removed. No reference material will be available to accounts who are created solely for the purpose of trolling. InterNations reserves the right to remove content in which we believe is harmful to us or our community standards.

- QAT."

This got me wondering why trolls bother to troll. What do they get out of this? Thoughts?