Krishna’s smile has so many meanings. Like in the BG 2.10., Krishna smiles because his dear friend has now become his disciple.
Gudakesa-Arjuna; the master of curbing ignorance, Prapantah-Arjuna; the chastiser of enemies is himself acting as an ignorant and chastising himself and so Krishna smiles
Krishna smiles to see Arjuna who is always so eager to fight is now so doubtful.
A single smile has so much depth……..