Hegelian-sounding aphorism of the day:
Leftism asserts the negation; liberalism negates the assertion.
Thus, the Left wants to destroy the heritage of the past, and so assert that the past must be negated. Liberals, on the other hand, simply deny that the heritage of the past should be asserted. (If you want an example, liberal anti-anti-communism is an obvious one.)
So far as I can tell, liberals and the Left act that way because they basically don’t like existence. When something exists it excludes things, and can’t be made into something else, and that seems intolerant, obstinate and even aggressive. The Left therefore wants to destroy whatever exists, because existence means limits, while liberals hope whatever exists will go away if they just stop supporting it.
All of which sounds a bit on the bizarre side, but it does make sense. After all, we distinguish reality from fantasy because reality resists our will, so on the face of it reality seems like a bad thing. To reconcile ourselves to it we need to believe that what we want and how things seem to us isn’t the whole story, that there is more to things than meets the eye, and that to understand the world you need to go beyond what you want and how to get it. What defines liberalism, the Left and modernity, though, is rejection of that view.