In The Night Thoreau Spent in Jail, why does Thoreau ultimately return to the human race?
In the "Production Notes" following the play it states that "Thoreau's decision to return to the human race is the shape, the parabola, of the play."
Thoreau's arrest was due to his not paying taxes, a specific passive protest against the Mexican-American War, which he saw as unjust and immoral. He originally traveled to Walden Pond to escape civilization and find his own goals and purpose in life, in tune with his Transcendentalist beliefs; Thoreau didn't believe that a man needs other men to survive and thrive. However, in arguing against the War and trying to convince others, Thoreau realizes that isolating himself in the woods does nothing to help the public-at-large understand his ideals; he cannot change public opinion by hiding.
HENRY: I may not be there at the pond place, Bailey. Seems to me I've got several more lives to live. And I don't know if I can spare any more time for that one.
(Lawrence and Lee, The Night Thoreau Spent in Jail, Google Books)
Thoreau comes to the conclusion that he has realized his goals at Walden, and he does not need to isolate himself any longer. Instead, he will rejoin civilization and help others attain the same goals; Thoreau will lead by example instead of ignoring humanity. In other words, abandoning humanity does nothing to help it; despite the revulsion at directly or indirectly supporting an immoral government, Thoreau will try to facilitate change from within by changing minds.