First of all the smiles of the Buddha are very constrained in most cases. It is not a wide smile but an introspective sort of subtle smile that I see on most representations. The point is that the Buddha knows he has escaped the wheel of Samsara, and that is why he is smiling. But he is actually smiling to himself, not to others, for he knows that others have not escaped yet. But he knows that others can escape because he has escaped and thus he is smiling to himself. It is the satisfaction of arrival. Every moment is that arrival which has been struggled for and attained. And so the smile is perpetual. But in the representations it means you can achieve this satisfaction yourself. And from the point of view of that arrival there is nothing to worry about any more. So it is assurance for the rest of us, and it is meant to spur us on to achieve the same state.
But of course it was realized that this was ultimately selfish, and so the Bodhisattva ideal was formulated to counterbalance the selfishness of the Buddha, however minute that might be, in as much as he entered parinirvana without taking us with him. So Bodhisattvas say they will wait to be the last to cross over into nirvana and enter parinirvana so that they can shower their compassion on living beings caught up in Samsara. Bodhisattvas tend to look determined. Determined to save all sentient creatures from delusion before they themselves give up reincarnation.
So the smile of the Buddha in light of the Bodhi Sattva ideal is his satisfaction that others will take care of the mortals he has left behind. It is the smile of a tremendous burden being lifted from his shoulders.
What people don’t get is that in Hinduism there was an infinite pressure on each person to be responsible for what ever they do. Reincarnation was devised to release the Gods from responsibility for the fates of humans. Buddha was not searching for release from the suffering of life alone, but the endless suffering of reincarnation,. When he achieved nirvana he actually realized that this wheel of samsara which put infinite pressure on each person, and caused them infinite suffering of continual rebirth in different forms forever was in fact an illusion. So the Buddha is smiling because he realized that Samsara was not real but illusory, and so there is no infinite pressure of reincarnation, but in fact reincarnation is merely the meaning of the habitas from moment to moment as quasi-causality continues to appear to flow in spite of emptiness being more fundamental.
Finally the Buddha is smiling because he realizes that even nirvana the opposite of samsara is also an illusion. there is no such thing as nirvana as any thing different from Samsara, these are nihilistic opposites and Buddha finds the nondual position between them, the middle ground where there is no extremes any longer. Emptiness of Emptiness means that the search for nirvana is a trick one plays on oneself, but this trick actually works, because the self gets caught up in this search to such extent that the whole self is invested in it, but when one realizes nihilistically that nirvana does not exist actually, but effectively it has in fact caused the self to disintegrate upon the realization, then one gets the subtle joke that is at the heart of existence. So the Buddha smiles at that joke he played on himself which was effective even though it was itself also an illusion.
And the Buddha smiles because he realizes that the way of Nirvana is not really the ultimate way, but it is the way of the Bodhisattva, which is ultimate, even though without him there could have been no Bodhisattvas. In other words if he had not postulated nirvana as opposite samsara, then it could not be realized that the ultimate position in Mahayana would be that of not entering fully into parinirvana but waiting for all the others. Of course it is going to be a very long time before all the others are saved, and in fact it is probably never going to happen, which means there will be a lot of Bodhisattvas hanging around going though Samsara with us. So the ultimate position is the escape of no-escape, just like the Koan collection is the gateless gate. Ultimately the realization is that Nirvana is not a door out of samsara, because neither nirvana nor samsara exist and the concretion of this realization is ultimately DzogChen which denies both as extremes and fabrications which in Tibet is seen as the highest teaching in Buddhism and was also not just in Buddhism but also in Bon (the Taoism of Tibet).
In effect the smile of the Buddha has infinite depth as we read into it the various interpretations of Buddhism though the ages. And ultimately the Buddha is smiling because of the subtlety of the Dharma that he has unleashed that has given rise to so many different viewpoints on Samsara and Nirvana over the centuries. The Buddha is satisfied with the opening up of so many deep ways of looking at existence beyond the illusions of Being. You can get lost in the infinite depth of that self-reflective smile that comes from satisfaction of the overcoming of the intrinsic and endemic dissatisfaction (dukha) of human existence which is transformed by the dawning of prajna.
Catch sight of that smile in yourself and claim your freedom from delusion.