All ppl doesn't have the same set of opinion, view, action and understanding. Ppl are like pendulum ball. some loves 2 follow in left, some in right and some in different zone. So, its hard too illustrate about ppl. :) :)

"Why is the [what?] so difficult between people?" A noun needs to follow "the". Do you mean, "Why is it so difficult between people?" or "Why is understanding between people so difficult to attain?" To correct Гаурав's answer... "Not all people have the same sets of opinions, views, actions or understanding. People are like pendulums: some love to swing left, some right, and some in another direction altogether. So it's hard to explain people's motivations." (The abbreviation of 'ppl' is not appropriate in this context.)