This Is a Certified Answer

Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
What was the reason behind both the neighbours for bringing up the wall? What did the wall divide? (Long Answer on this question)

In this poem "Mending Wall" two neighbours always met for bringing up the wall which separated their property. In this poem Frost describes two different mentalities. 
The wall were subjected to natural calamities and the boulders which constitute the wall fell off but soon after they gather and put those boulders again at the same place thus creating the wall again.
It seems as if the nature too wants to break the man-made walls.
The presence of wall does not have a great impact on the relationship of the two neighbours. The presence of wall restrict them to stay more to themselves rather than communicate and share ideas. They remain as individuals. They just meet when they want to mend the wall.