Density functional theory computations and block-localized wavefunction analyses for 57 hydrogen-bonded base pairs document excellent linear correlation between the gas-phase association energies and the degree of aromaticity gain of paired bases (r = 0.949), challenging prevailing views of factors that underlie the proposed electronic complementarity of A·T(U) and G·C base pairs. Base pairing interactions can polarize the π-electrons of interacting bases to increase (or decrease) cyclic 4n + 2π electron delocalization, resulting in aromaticity gain (or loss) in the paired bases, and become strengthened (or weakened). The potential implications of this reciprocal relationship for improving nucleic acid force-fields and for designing robust unnatural base pairs are discussed.