I like Thomas Sowell's theory (or maybe it's more of an explanation than a theory -- he lays it out in his book, The Quest for Cosmic Justice, which is a great read) which is that for the left, it all boils down to Cosmic Justice. They want a government with a conscience, with a willingness and ability to correct the universe's cosmic unfairness. Of course, this requires the government to be very powerful, but -- and here's where Sowell is really insightful -- it also requires that the government abandon traditional justice, since traditional justice does not necessarily lead to cosmic justice and in fact frequently acts as an obstacle to the administration of cosmic justice.
An example...
It is cosmically unjust for one person to be born into wealth while another is born into poverty. The Left wants a government that will right this wrong by taking money from the first person and giving it to the second. In so doing, the government will have meted out cosmic justice, but it will have violated the principle that a person should not be robbed of their wealth -- a part of traditional justice.
Another...
The existence of inequality between the races is cosmically unjust. Leftists desire a government that is empowered and willing to rectify this inequality through such actions as the enforcement of race-based quotas. In so doing, the government has meted out cosmic justice, but it has violated the principle that people should not be treated differently because of their skin color, which is part of traditional justice.
And so on.
The irony is that all this meting out of cosmic justice never really solves problems and usually just creates more suffering. Ultimately, it is voluntary cooperation between people (along with love and forgiveness and bravery and integrity and stuff like that) that solves humanity's "cosmic" problems, not governments with God-like power to force cosmic justice onto people.
I guess Sowell's angle is really a spin on the means versus ends thing, which is nothing particularly new, but his "cosmic justice" packaging is pretty neat.
I have read most of his book on The Anointed and thought it was on the mark now I can see that I ought to find time for the Cosmic book!