When a gas is adsorbed on a solid surface, the movement of the gas molecules becomes restricted. This leads to a decrease in the randomness or disorder of the system, meaning the entropy decreases ($\Delta S$ is negative). For a process to be spontaneous, the Gibbs free energy change ($\Delta G = \Delta H - T\Delta S$) must be negative. Since $\Delta S$ is negative, the term $-T\Delta S$ becomes positive. To make the overall $\Delta G$ negative, the enthalpy change ($\Delta H$) must be negative, which means the process must be exothermic. Thus, adsorption is generally exothermic because entropy decreases.