What is the theme of the Rudyard Kipling's poem "If"?

While there are several themes in Rudyard Kipling's "If," an overriding theme is the striving for balance on the way to spiritual enlightenment--the Middle Way of Buddism, or the Golden Mean of Aristotle. 

In Shakespeare's Romeo and Juliet, Friar Laurence cautions Romeo, 

These violent delights have violent ends,
And in their triumph die, like fire and powder (2.6.9-10)

And, in Kipling's "If" the father advises his son of the same temperance, encouraging his child to strive for the Middle Way. This perfect balance is achieved by restraining from excess:

If you can dream—and not make dreams your master;   

If you can think—and not make thoughts your aim; 

Much as in the Proverbs of the Bible, Kipling makes effective use of parallel structure and repetition, which both serve to reinforce his idea of the need to be able to both ignore doubt and make allowances for doubt. 

If you can wait and not be tired by waiting, 

Or being lied about, don’t deal in lies, 

Certainly, too, this repetition and parallel structure enhance the effectiveness as well as the theme of the didactic poem "If." This theme relates to the teaching of Aristotle with the Golden Mean, as well as alluding to a basic teaching of Buddhism with the Middle Way.