Why is Lord Krishna always smiling?

As the lord explains in the the Bhagvad Gita, continuosly through his various energies he perfects the science of yoga and the supreme truth thus manifests in his personal form. This perfection is realisation (knowledge) of the absolute true self ( shakti) (as the lord revelaled in his universal form) and equates to nirvana/bliss. So the lord projects nirvana which is utter bliss and happines marked by the smiling face. Indeed mankind is encouraged to follow this path of perfection and they too will always be smiling.

Krishna always smiles because he got his mind thoughtless completely. Absolute thoughtless mind is directly attached to Almighty that is The Parmatma. The Parmatma is in every particle of the universe. So Lord Krishna was able to create or modify surroundings, atmosphere you can say, suitable for holy souls. Now holy souls are those who have only one aim..feel "The Parmatma". Lord Krishna shows path to those holy souls, he removes hurdles from this path anyhow..n he knows he can. He was doing these tasks constantly. To get the answer of this question why he is always smiling, you have to experience on your own. Try to be thoughtless, Leaving all feelings, materials, stress leaving behind..with just one aim..you'll get the answer.