The spontaneity of a reaction is determined by the Gibbs free energy change ($\Delta G$). For a cell reaction to be spontaneous, the free energy of the system must decrease, which means $\Delta G$ (or $\Delta G^{\circ}$ under standard conditions) must be negative. The relationship between standard Gibbs free energy and standard cell potential is given by the equation: $\Delta G^{\circ} = -nFE^{\circ}_{cell}$. Since $n$ (number of moles of electrons) and $F$ (Faraday constant) are positive values, $\Delta G^{\circ}$ will be negative only when the overall cell potential $E^{\circ}_{cell}$ is positive.