Going undercover - the worst job ever?
So you put yourself in dangerous situations with dangerous people, spend loads of time away from your family (and Pistone's breaks down in the film), have to not step in as other people get beaten up or killed, get transformed by the experience (e.g. the way he begins to talk in the film)... and then betray all these people that you've spent months getting 'close' to (if they really are close, like Lefty and Donnie are) and have the entire mafia against you so you have to spend your life in witness protection and quite possibly get found and be tortured/die/have your whole family die.
How much does he get paid for this? How can it ever be worth it??
Also, there's no way you can spend this much time with people and not get close to some of them, which makes the eventual 'betrayal' that much harder. I really felt for Lefty at the end.
"I...drink...your...milkshake!"