The left is afflicted with cognitive dissonance. It drives them crazy and they're not even aware of it.
The wackypedia definition of CD is "the feeling of discomfort when simultaneously holding two or more conflicting cognitions: ideas, beliefs, values or emotional reactions". It is the Yang to the natural Ying of harmony. Voices singing in harmony, in concord, and in unison are naturally simpatico. Forces working against one another create friction and discord because they are naturally antagonistic.
People working towards common goals and aspirations - even when they are from diverse backgrounds - blend and strengthen. People working at cross-purposes invariably find themselves in direct opposition to others and that is where antipathy and conflict occur.
We can "celebrate diversity", but only from a position of cultural harmony and unity. Multiculturalism is suicidal madness and the perfect prescription for the death of the host culture. (Yes, I know that I am preaching to the choir here).
The left knows these truths but, like moths drawn to a candle, cannot stop themselves from pursuing the pseudo-intellectual nihilism that is known as Multiculturalism.