In group power dynamics. Once a person identifies with a group, and makes the beliefs of that group part of their identity, then they will fight any threat to it. Since there are just two parties you are forced to choose one or the other.

The single greatest thing the American people can do from this moment on is to stop hating each other for political beliefs, put that aside, and just talk without expectation or trying to convince someone. Just talk. America has let political identity supersede all else.