What was in the mind of the poet when he wrote the poem "The Road Not Taken"?

When asked if he would reveal the intended meanings in his poems, Robert Frost replied, "If I wanted you to know I'd had told you in the poem." So, no one really knows the hidden meaning of "The Road Not Taken." 
One thing that is known about this poem by Frost is that he composed it shortly after returning home in the United States from his trip to England, where he accompanied his fellow poet and friend, Edward Thomas on walks. This friend would often take Frost with him on woodland strolls where Thomas would be very indecisive about which path to take. He would worry that, perhaps, he had chosen the wrong path as another might have had more fauna and flora.
At any rate, Edward was rather indecisive. So, he certainly fits the character of the poem's speaker who suffers in the second stanza as he deliberates over his decision to take the "other" path that was "just as fair"--

And having perhaps the better claim,Because it was grassy and wanted wear;Though as for that the passing thereHad worn them really about the same

Still, the power of poetry is that it speaks to the heart, and there are always various meanings that different people derive from poems. So, while Frost may have had some fun writing about this friend, he also may have intended for his readers to think of the metaphoric meaning of his poem as one about life's choices and their profound affect upon people. After all, in 1961 Frost did say that "The Road Not Taken" is “a tricky poem, very tricky.”
http://www.pbs.org/wgbh/poetryeverywhere/frost.html