In A History of Western Philosohy, Bertrand Russell wrote of Leibniz that

His best thought was not such as would win him popularity, and he left his records of it unpublished in his desk. What he published was designed to win the approbation of princes and princesses. The consequence is that there are two systems of philosophy which may be regarded as representing Leibniz: one, which he proclaimed, was optimistic, orthodox, fantastic, and shallow; the other, which has been slowly unearthed from his manuscripts by fairly recent editors, was profound, coherent, largely Spinozistic, and amazingly logical. It was the popular Leibniz who invented the doctrine that this is the best of all possible worlds (to which F. H. Bradley added the sardonic comment "and everything in it is a necessary evil"); it was this Leibniz whom Voltaire caricatured as Doctor Pangloss. It would be unhistorical to ignore this Leibniz, but the other is of far greater philosophical importance.

and Russell seems to think that "best of all possible worlds" is the shallow public theodicy, and "most existent" is the private theodicy, and they are not the same thing - since privately (according to Russell's account), Leibniz speculated that the world which gets to exist is the one which has the most entities in it (maximum number of entities logically capable of coexisting). But then Russell also writes that Leibniz may have considered this a sign of God's goodness - it's good to exist, and God makes the world with the most possible things... I am much more sympathetic to Nietzsche's metaphysics, as described in the posthumous notes collected in The Will to Power, and his skeptical analysis of the psychology behind philosophies which set forth identities such as Reason = Virtue = Happiness. Nietzsche to my knowledge did not speculate as to why there is something rather than nothing, one reason why Heidegger could see Nietzsche's ontology as the final stage in the forgetting of Being, but his will-to-power analysis is plausible as an explanation of why beings-who-happen-to-exist end up constructing metaphysical systems which say that to be is good, and to be is inevitable, so goodness is inevitable.