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The offset of high redshift star-forming galaxies in the [OIII]/H$\beta$versus [NII]/H$\alpha$ (O3N2) diagram in comparison with the local star-forminggalaxy sequence is now well established. The physical origin of the shift isthe subject of some debate, and has important implications for metallicitymeasurements based on strong lines at all redshifts. To investigate the originof the O3N2 offset, we use a sample of ~100,000 star-forming galaxies from SDSSDR12 to probe the empirical correlations between emission line diagnostics andmeasurable galaxy physical properties. In particular, we examine how surfacedensity of star formation, ionization parameter, nitrogen-to-oxygen (N/O)ratio, and stellar mass drive position in two key diagnostic diagrams: O3N2 and[OIII]/H$\beta$ versus [SII]/H$\alpha$ (O3S2). We show that, at fixed[OIII]/H$\beta$, galaxies falling closer to the high-redshift locus in O3N2have higher star formation density, stellar mass and N/O ratios. We also find atight correspondence in the distributions of stellar mass and N/O in thediagnostic diagrams. This relation, spanning a range of galaxy evolutionarystates, suggests that the N/O-$M_{*}$ relation is more fundamental than theN/O-metallicity relation. We argue that a tight N/O-$M_{*}$ relation iswell-motivated physically, and that the observed correlation of N/O with O/H inthe local universe is primarily a reflection of the existence of themass-metallicity relation. Because the mass-metallicity relation evolves muchmore rapidly with redshift than N/O-$M_{*}$, the N/O ratios of high redshiftgalaxies are significantly elevated in comparison with local galaxies with thesame gas-phase O/H. The O3N2 shift and elevated N/O ratios observed in highredshift galaxies therefore come about as a natural consequence of theN/O-$M_{*}$ relation combined with the evolution of the mass-metallicityrelation.

A Tight Relation Between N/O Ratio and Galaxy Stellar Mass Can Explain the Evolution of Strong Emission Line Ratios with Redshift