Some reasons people engage in trolling behavior include:
Unless trolling is done in the most niche forum of humor I just think it's like the dumbest thing ever. I think it's a crazy waste of time for the troll and those getting trolled and it just says a lot about the person one is if they choose to spend their day hurting or bringing down others. Sometimes it's just a by-part of someone being funny in a game or playful with buddies, but most often that's not the case. It's just weak and to bully from behind the key board shows how little strength and courage the bully really has. It's something internal - like a God complex - to need to feel satisfaction from messing with others (especially innocent users).