What causes resentment between classes is lack of mobility. This, IMO, is caused by government regulations and a banking system that hurts the middle class and below and benefits the people at the top.
We have a banking system where the federal reserve can make money out of thin air and inject it into banks. This shifts purchasing power to people in finance.
Regulations like minimum wage and higher taxes on larger income make it hard to hire the next marginal employee. Minimum wage prices out low skilled workers, who would like to work and obtain on the job experience needed to climb the ladder. High taxes makes it more expensive for business owners to hire someone. I'm not talking about large businesses like Walmart (they actually support minimum wage). Small businesses are going out of business, or are just not able to start with increased costs of doing business.
This is a vicious cycle where government intervention leads to rich getting richer, and the poor being more dependent on government programs and finding it harder to move up the ladder. So to fix this people beg for more government intervention, which worsens the problem...and on and on it goes.