When it comes to pension systems there are two types; funded and unfunded. Funded retirement systems are ones which you must put money into such as a savings account or a 401k plan. Unfunded systems are ones such as social security where tax money is used to pay for the pensions of the already retired. The problem is unfunded systems ultimately lead to a self perpetuating conflict between generations.
Unfunded systems ultimately lead to the retired voting for better benefits which leads to higher tax burdens on those still working. I think you can see where the resentment generates. The article is a good read.