Gautama sits under the tree and achieves enlightenment and in that moment is given a choice to remain in nirvana or return (as all hero’s must) to teach what cannot be taught. Gautama knew that most would relate to the teachings as if they were the destination mistaking the map for the  territory. (even as all of the teachings warn not to do that)
I’ve often thought that such a paradox would be enough to boot one right out of nirvana and into frustration. Still the Buddha agrees to return and teach as a enlightened one and I wonder if this is why he laughs.

I’ve also wondered about all those Zen stories where the Buddha teaches and someone hearing becomes immediately enlightened in that moment. I imagen this now enlightened person looking into the Buddha eyes to mark the moment – maybe the Buddha gives him a wink or some other form of recognition… but then the Buddha winks at everyone. I imagen this now enlightened person waking out of the temple, perhaps in a state of contentment, maybe even bliss and then stepping in dog poop.  I wonder if this person remains enlightened? I wonder if this is why the Buddha is so often depicted as laughing.

“Before one studies Zen, mountains are mountains and waters are waters; after a first glimpse into the truth of Zen, mountains are no longer mountains and waters are no longer waters; after enlightenment, mountains are once again mountains and waters once again waters.” Dogen