In Proof of Stake the people that hold the most coins sign the blocks that will be part of the block chain.
The theory is that people that have the most to lose from a 51% attack will do their best to prevent it.
But, if you have the most money, and are signing the blocks, don't you have the most to gain by executing a double spend attack, and nothing to lose if it fails?
Yes
this is the "nothing to lose" issue with PoS..
If you are going to double spend on PoW and fail, you've lost a bunch of hashing power meaning lost $$$.
Try the same on PoS and if you fail nothing is lost.. in other words: you have nothing to lose.