We analyze possible causes for chirality selectivity in catalytic synthesis of single-wall carbon nanotubes. Our analysis is based on the calculation of aromaticities (as a clear indication of reactivity) for closed-ended carbon nanotubes of a given chirality as a function of the nanotube length. We found that arm-chair nanotubes have variable reactivities as a function of their length, whereas chiral nanotubes (those most frequently obtained in the CoMoCAT process) show strong stable reactivity that remains constant as the nanotube growths, and other chiral nanotubes (closer to the zigzag type) have borderline reactivity. Our results show how the aromatic character varies from the open end (where the nanotube grows according to the root growth model), to the close end. Also, we investigate the influence of the metal cluster-nanotube interaction on chirality selectivity.