I think there's a number of reasons, but one of them might be that some people believe that the gap between the rich and poor/middle class continues to grow more and more. They see this as a bad thing, because they would prefer everyone growing at similar rates.
These people forget that if someone is rich, assuming every dollar is equal when viewed at a societal level, then it means that the rich person did more to benefit others. Some believe that this means they took more from others and left them with less. The only reason we enter into an agreement to do business, even if it's just buying an apple, is that we are better off. I don't go and purchase groceries if I end up worse than if I had held onto the money. If someone is rich, it is their money. (Even if it was inherited, it was the person who earned it's right to determine what to do with it - if they want to give it all to someone, that is their own right.)
I don't have a complete answer, but I think that it should be just the opposite - we should be happy that people are rich. If 1 doctor decides to work twice as much as someone else, and ends up rich, it's because he helped a lot more people than the other doctors. We should be thankful he was there to take our money, because he gave us good health. If someone gets rich off of renting apartment units, we should be thankful, because they were there to take our money. I think people forget that money alone is nothing, you rely on the very people you despise to turn that money into value.
The argument is often that they inherited it and it goes back to what I said earlier - someone, at some point, started with nothing. At some point, we had nothing at all. If anything, we have it easier today than 300 years ago, because only a fraction of our society's resources are used for food. The opportunity to become rich is much greater today than it was before.
I'm not rich by any means at all and just finished my undergraduate degrees, but I'm thankful for the rich people having done their part. Without their efforts, we'd be much worse as a society and there wouldn't be opportunities for people like me. On the whole, every time you earn a dollar, it's because you improved someone's life. They said, "Here, take my dollar and perform this function for me, because my life is made better." In turn, I either spend the dollar or hold onto it until I find something that will make my life better. For some, it could be a deferred use - so deposit it in the bank account and the bank will lend it out, or invest in the stock/bond markets, etc. so that I can claim it in the future and spend it.
I think we should be happy and thankful for the rich - it was their efforts that made our economy possible. We may have had a few bad years, but again, a few bad years out of 50 doesn't make it a bad system. In fact, compare the US to other countries and the short time period in which we've made progress and it will be easy to see that we have a system that works. On the whole, it rewards people who create value.