The attraction between π-conjugated planar electron donor and acceptor molecules that form many stable charge-transfer (CT) complexes has been explained by quantum chemical CT interactions. However, its fundamental origin remains unclear. Here, we elucidate the mechanism of CT complex formation through potential energy map analysis of TTF–CA and BTBT–TCNQ, which are representative examples of CT complexes composed of large extended π-conjugated electron systems. This analysis was conducted using energy decomposition of intermolecular interactions via symmetry-adapted perturbation theory (SAPT) with coupled cluster calculation. We found that the primary source of attraction between donor and acceptor molecules is due to dispersion forces, with electrostatic forces also playing a significant role, while the contribution of the induction term involving CT interaction is very small. Additionally, we demonstrate that the highly directional nature of the exchange repulsion forces, in conjunction with the attractive dispersion and electrostatic forces, is crucial in determining the intermolecular arrangements of actual CT crystals. These findings are key to understanding the unique structural and electronic properties of π-conjugated CT complexes.