The answer to that question is obviously complicated. I do know a few things, though. First, most Americans are decent people who just want to provide a good life for themselves and their family. Second, the nature of our partisan politics contributes to the divide.
Sure there are a lot of factors at play, socio-economic, cultural, racial, historical, and parochial divides all play there part. Despite the complexity of the issues, the cause is pretty straightforward, and it is related to the role the two major parties play in it.
In the last election cycle, Democrats did not contest 1,479 state legislative races. These uncontested seats were largely in rural areas. The result, Democrats, did not seek to represent 117,058,570 Americans in largely what is known as "Red America."
In the last election cycle, Republicans did not contest 1,442 state legislative races. These uncontested seats were largely in urban areas. The result, Republicans did not seek to represent 117,696,601 Americans largely across the urban areas.
The two major parties combined do not contest 2,921 state legislative seats out of 7,383 races. That is a full 40% of all these districts elections. Combined they do not seek to represent more than 234,000,000 constituents; is it any wonder we are so divided?
It is not that urban America and rural America no longer have any shared values. The perceived divide between "Red America" and "Blue America" exists due to the same reason so many relationships fail...we have stopped talking to each other.