Maybe "connectedness" contributes to the partisanship.
The most stable societies often seem to be the ones with the least diversity. It seems like the fewer the internal differences among the population, the fewer reasons to be partisan -- the other guy looks like you, speaks like you, prays the same, eats the same, lives the same.
Connectedness makes people aware of differences -- the other guy looks different, talks different, prays different, eats different, lives different.
Something about humans makes the other a competitor or an enemy.