"Buddha" is the designation for a person who comes to the peak of sentient evolution, beyond the stages and states of humans and even gods. It is defined as "awakened" from the sleep of misknowledge, and as "blossomed" into the omniscience and competence of universal compassion. This stage of being has many other names: "Bhagavan" - "Glorious," "Tathagata" - "Transcendingly Realized," and most important in our context today, "Sugata" - "Blissful," sukham - gata, "having become bliss". Such a being is described as The Buddha's Smile having three bodies, bodies of Truth, Beatitude, and Emanation. In any sort of Buddhism, it seems beyond question that "Buddhahood" is a synonym for supreme happiness. To state my thesis succinctly, from the Buddhist perspective enlightenment is happiness. So naturally a Buddha smiles.
In the Individual, Monastic, or Early Buddhist Vehicle, Buddha is called Sugata, Blissful One, he is said to have attained both Nirvana and Parinirvana, all suffering blown out and utterly blown out, to have reached the supreme happiness (paramasukha), to have gone beyond the Gods in joy, to have become the God of the Gods. In the Universal, Messianic, or Social Buddhist Vehicle, Buddhas are said to enjoy a Beatific Body (Sambhogakaya), to experience nonduality of Nirvana and the world, and are depicted as theistically capable of producing whole universes of bliss, buddhaverses (buddhakshetra) of Bliss (Sukhavati), Intense Delight (Abhirati) and so forth. And in the Lightning, Diamond, or Apocalyptic Vehicle, ultimate reality as expeiencedin enlightenment is described as bliss-void-indivisible (sukhashunya-avinirbhaga). So the overall Buddhist view must be that supreme enlightenment and true happiness are one and the same.
Misapprehensions
If this is so, then why is Buddhism considered a pessimistic, nihilistic, even morbid religion or philosophy? And this not only by moderns in the West, but by some Brahmins at various times in India, some Mandarins at various times in China, and other authorities all over countries where Buddhism flourished for millennia? I think the essence of the answer lies in the corollary of the thesis "enlightenment is happiness"; namely, that ignorance is misery.