There are a number of possible reasons why Congress is so partisan today.
One factor is that Americans are becoming more partisan. It was once the case that many Americans were relatively moderate. Today, Americans are becoming more extreme. There are more strong liberals and more strong conservatives than in times past. This means that American voters are no longer willing to compromise much. They want representatives who will take hard lines and will stand up to the other party.
A second factor is that years of enmity have built up between the two parties. Both of the last two presidents have been extremely controversial. Both President Obama and the second President Bush have been vilified and hated by the opposing party. The opposing party has done everything it could to stop both presidents from getting things done. This has helped create a situation where the legislators from the two parties have developed grudges against one another.
A third factor, according to some political scientists, is the fact that members of Congress no longer build personal relationships with one another. In the past, members would socialize across party lines. Today, members do not really have the time to socialize. They are too busy raising money and going back to their districts and they do not spend time building relationships that could transcend partisan differences.
In my view, the most important factor is that there are very few competitive Congressional districts any more. Because of gerrymandering and because of changes in residential patterns, most districts are “safe” for one party or the other. This means that candidates who want to represent those districts do not need to try to appeal to members of the other party. Instead, they have to worry about primary challenges from more extreme members of their own party. They have to prove that they are the most radical candidate so as to get the voters (who, as noted in the first paragraph, are more extreme) to elect them. This means that the people we send to Congress are more radical in ideological terms.
So, we have strongly conservative and strongly liberal members of Congress, backed by more conservative or more liberal voters. These members have become angrier at the other party and have fewer social ties to the other party. All of these things help to make Congress more partisan.