To my ear, unless "essence of human" is a fixed expression like "essence of rose," the sentence should begin with "The." (The quality of mercy is not strained.)
The essense of human relationships is volitility, maybe?
It absolutely needs the "The" and leaving "human relationship" in the singular is quite odd. But ending it with an adjective instead of a noun was also quite odd. The essense is ... I expect a noun.