Liberals think that advantage, especially economic advantage, is unearned and undeserved. For example, an advantaged (rich) person got his money due to illicit practices, exploitation, inheritance and/or luck. Since all of these means are improper to a liberal, the advantaged person has no right to his position. As a corollary, a disadvantaged person has every right to usurp the position of the advantaged (i.e., start at the top). In fact, a disadvantaged person may see such usurpation as a noble act.
In Marxism, all inequities are the result of class differences ... social constructs, not human failings. "Fix" the society and you erase the inequities. The corollary of course is that no one "deserves" more than any other, regardless of their abilities, ethic, or diligence. They should simply revel in their ability to contribute more to the Collective since, in the edenic Marxist vision, no status accrues to material wealth anyway.
And you're right. It was Yeats, not Keats. Unpardonable sin for an English major.