There are many reasons for the reason while the Buddha is alway's smiling. The main reason is that he has liberated himself from suffering. When someone attains enlightenment on a permanet basis he sees the end of suffering permently and expirences freedom from suffering. When someone is practicing meditation to gain this kind of freedom he will see the path to this enlightenment, the path to Nirvana and will set his mind one pointedly towards ending suffering, this one pointedness of mind produces a feeling of happiness in his mind, which produces finer material properties in his body, these fine properties produce a small smile.