The trihydrogen cation, H3+, is unique in the Universe. It serves as the primary proton reservoir, driving essential astrochemical reactions, and it functions as a thermostat for giant gas planets. H3+ has also remarkably low photodissociation rate, explained by its exceptionally high first electronic excitation energy (19.3 eV). Herein we reveal the key factors behind this high energy: (i) aromatic stabilization in its electronic ground state, (ii) antiaromatic destabilization in its first excited state, and (iii) a high nuclear-to-electronic charge ratio (+3 vs. -2). Through comparisons with analogous pi-conjugated carbocations, we find that ground state aromatic stabilization plus excited state antiaromatic destabilization raise the excitation energy of H3+ by 4.8 - 6.0 eV. Only with this increase can it fulfil its unique functions in space.