Unfortunately, the contrast that memory presents with more recent events is one of the reasons why it comes up so often.
Of course, the next point that often comes up involves blaming one side or the other for the increase in polarization. Even if such arguments are true, correctly blaming people for what they've done wrong isn't going to foster unity.
Even as we disagree, we need to learn how to acknowledge why our opponents believe what they do, what they do right, and where we agree. Even if we don't agree in anything other than wanting the United States to be the best country it can possibly be, that's still better than nothing.
It's not going to make us always agree. It's not going to make the issues we argue about any less important; sometimes trying to work on unity is going to have to get in line behind advocating for people's rights and freedoms. And one side or the other can destroy the whole effort unilaterally; both sides have to agree that it's worth it and put in the work to make it happen.
If we can do it, though, I think we can look forward to a better country.