When you increase an individual's wealth, he becomes less dependent on government, and his attitude towards government changes. Socialists can't allow that, for it erodes their fundamental principle that social justice can only be achieved when important segments of the economy are under government control.
You can also see this in the way that each side refers to tax cuts. Those on the left say that the government can't afford to give that money away. The right see it as not taking somebody's hard earned money from them in the first place. In other words, the Democrats think that they have more right to your paycheck than you do.