Robert Frost's poem

I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I--
I took the one less traveled by,
And that has made all the difference.

- From " The Road Not Taken" by Robert Frost

I have read this poem several times and always wondered about the "sigh" for taking the road less travelled. Finally I found the answer in one of the sites about Robert Frost's poetry:

The inspiration for it (The Road Not Taken) came from Frost’s amusement over a familiar mannerism of his closest friend in England, Edward Thomas. While living in Gloucestershire in 1914, Frost frequently took long walks with Thomas through the countryside. Repeatedly Thomas would choose a route which might enable him to show his American friend a rare plant or a special vista; but it often happened that before the end of such a walk Thomas would regret the choice he had made and would sigh over what he might have shown Frost if they had taken a "better" direction. More than once, on such occasions, the New Englander had teased his Welsh-English friend for those wasted regrets. Disciplined by the austere biblical notion that a man, having put his hand to the plow, should not look back, Frost found something quaintly romantic in sighing over what might have been. Such a course of action was a road never taken by Frost, a road he had been taught to avoid. In a reminiscent mood, not very long after his return to America as a successful, newly discovered poet, Frost pretended to "carry himself" in the manner of Edward Thomas just long enough to write "The Road Not Taken". Immediately, he sent a manuscript copy of the poem to Thomas, without comment, and yet with the expectation that his friend would notice how the poem pivots ironically on the un-Frostian phase, "I shall be telling this with a sign". As it turned out Frost’s expectations were disappointed. Thomas missed the gentle jest because the irony had been handled too slyly, too subtly.

A short time later, when "The Road Not Taken" was published in the Atlantic Monthly for August 1915, Frost hoped that some of his American readers would recognize the pivotal irony of the poem; but again he was disappointed. Self-defensively he began to drop hints as he read "The Road Not Taken" before public audiences. On one occasion he told of receiving a letter from a grammar-school girl who asked a good question of him: "Why the sigh?" That letter and that question, he said, had prompted an answer. End of the hint. On another occasion, after another public reading of "The Road Not Taken", he gave more pointed warnings: "You have to be careful of that one; it’s a trick poem – very tricky". Never did he admit that he carried himself and his ironies too subtly in that poem, but the circumstances are worth remembering here as an illustration that Frost repeatedly liked to "carry himself" dramatically, in a poem or letter, by assuming a posture not his own, simply for purposes of mockery – some times gentle and at other times malicious.