The story is that the illumined one came to the edge of this vale of tears -- this realm of duality -- and was on the verge of stepping into the dimension of perfect unruffled bliss, but he then paused and chose to return. It was, the story goes, compassion that brought him back and led to his famous vow that he would not separate himself from this realm of struggle and difficulty until every sentient being was liberated from suffering.
Well, that's close. He was indeed compassionate, and that has something to do with the story, but actually it didn't happen quite the way it has been handed down. Being the most enlightened entity of his day he came to realize that he simply couldn't take that final step by himself, even if he wanted to, for the simple reason that on some deep level we are all one and therefore so long as anyone suffers we all suffer. Furthermore he also saw that realm of perfect bliss in the unmoving center of all things exists as the unmoving center of a wheel, the spokes and rim of which are meant to move. Hence, the goal of spiritual development is not to escape this wheel which is the world of becoming, but to transform our manner of being in it. His actual vow at that time was to hold off doing anything very decisive until he had a few more centuries to think it through more carefully. Later he came to think of this as his First, or his wishy-washy vow. It was not a vow he talked a great deal about, realizing perhaps that it didn't have a nice ring to it. "I vow to bide my time and think things through a bit more." It was not the sort of thing that makes for a good story or lofty poetry.
To his credit he tried to explain everything to the scribes of his day. He mentioned his vow, which he admitted, really wasn't so much a vow as it was an interim plan to tide him over until he could think of something better. He said he didn't mind if they chose to call it a vow, but he then tried to explain about how he couldn't have gone anywhere in any case, unless everybody was ready to go with him, which was really the important point. The scribes, however, were much like the scribes of every tradition and of all times, and they simply could not or would not hear any idea that varied in the least from what they already thought. So they wrote down an account that seriously misrepresented what he told them. In time the Bodhisattva came to realize that, although he could live without eating or drinking, and could move mountains, and could understand mysteries deeper than the oceans, and could heal the sick, and could work all manner of miracles, he could not change the mind of a scribe.
So he withdrew from the realm of the respectable.
He became a wanderer once again -- a vagabond -- a man without a home either in this realm or the next. He wandered through many incarnations determined to understand why humanity could not realize it's true nature. He became mentally ill and spent time in state hospitals. He became retarded and spent time in institutions for people who could not tie their shoes. He became a criminal and spent time in prisons. He became a bag lady and an illegal alien and a beggar. He spent several incarnations living the lives of different sex perverts, and he enjoyed their pleasure and suffered their condemnation. And he said to himself, "what shall I do for this humanity that cannot seem to understand that they share a single wound?" And that was when he made his vow.
He decided to cast his lot with the riffraff, the outcasts, the slime-balls, the ignorant, the deformed, and the demented, and he vowed that he would never step back into the circle of acceptable humanity again until every last broken and despicable specimen was welcomed back with him. This was a vow he was happy with. "For the whole problem is that we draw a circle that can leave somebody out," he said. "That's the First Noble Truth." He dictated this to the scribes, and again they couldn't hear.