Torchlight II has a wealth of New Game+ content. Usually a game unlocks New Game+ after you run through once, but Torchlight II can go all the way up to New Game+++++, with each "+" making the game more difficult. The challenge is certainly welcome, but a bug discovered in the game is making things just a tad too difficult.
If you save or quit while playing a New Game+ run-through, the next time you start the game up, it will move up one level to New Game++. Obviously the same thing happens if you're running New Game++ or anything higher. In other words, the game will keep getting more difficult every time you quit.
There's no fix for the bug right now, but hopefully one is on the way. Unless you plan on playing through in one sitting, you might want to skip New Game+ for now!