I don't know when the poem was originally written, but the book with Cooney's illustrations was originally published in 1979, so clearly part of the same cultural zeitgeist. I will be ME and no other! To thine own self be true!

It's maybe a long (long, long) hangover from the pro-conformity messages of the 1950s? A more general cultural feeling that the discovery or crafting of the Self is the highest human good? Maslow's hierarchy of needs with self-actualization on top.