Why do authors vary sentence structure in their writing

Hurry i am on the semester test which theme is evident in this excerpt from robert frost's "mending wall"? but at spring mending-time we find them there. i let my neighbor know beyond the hill; and on a day we meet to walk the line and set the wall between us once again. we keep the wall between us as we go. to each the boulders that have fallen to each. and some are loaves and some so nearly balls we have to use a spell to make them balance: "stay where you are until our backs are turned! " we wear our fingers rough with handling them. oh, just another kind of out-door game, one on a side. it comes to little more: there where it is we do not need the wall: he is all pine and i am apple orchard. my apple trees will never get across and eat the cones under his pines, i tell him. he only says, “good fences make good neighbors." spring is the mischief in me, and i wonder if i could put a notion in his head: "why do they make good neighbors? isn't it where there are cows? but here there are no cows. before i built a wall i'd ask to know what i was walling in or walling out, and to whom i was like to give offence. . " a. the human desire for material gain b. the influence of financial constraints c. the positive effects of friendship d. the uncertain nature of human relations e. the futility of human yearning