The last stanza is much debated but there are a few clues as to its meaning. Firstly, the narrator tell us with a 'sigh' which seems to indicate regret. We know that the narrator is now speaking a long time later as he refers to the choice of path he made as being in the past. ""I shall be telling this with a sigh
Somewhere ages and ages hence:"
This gives the reader a sense of the future. The narrator seems to indicate that he chose the less traveled road and that, that one choice in his life had made all the difference.
Some critics consider the last stanza to be a celebration of non conformity meaning that because the narrator took the path less traveled in other words the unusual way - this action has made and defined even his future life.
Other critics tend to see this poem as sad. The 'sigh' at the beginning of the last stanza indicates remorse and the distance between the choice of path and the time of narration indicates an older person looking back on life.
To answer the last part of the question. If you accept the first interpretation of the poem (non conformity) then the reason he chose the less traveled road is to be a non conformist. If you accept the second interpretation, then you do not know why the narrator took the less traveled road - only that he regrets it.
When the traveller in Robert Frost's poem, The Road not Taken arrives at a fork in the road in the 'yellow wood', he is caught in a moment of choice. He chooses to take 'the one less travelled by'. This choice seems to suggest the non-conformity of a strong individualist who always gambles for the more untrodden ways of life. But the 'sigh' heaved by the traveller may also indicate an element of regret. May be that by adopting 'the less travelled' of the two roads, he has made his life so different, and it's a difference for the worse. The simple literal meaning of the poem thus gets subjected to an ironic implication with regard to the making of a wrong choice in the journey of life.