Why do people bring up the past in arguments?

Usually because the old hurt was never finished. Resentment that wasn't dealt with when it happened waits for an opening, and a small fight about something else is an opening.

Research by Kassandra Cortes and Anne Wilson points to one thing that makes it more likely: how recent a hurt feels, which isn't the same as how recent it is. In their studies, people were more likely to have old hurts come to mind in new, unrelated moments when those hurts still felt close, and that habit predicted worse outcomes in the relationship.

Throwing every old grievance into one argument: everything but the kitchen sink. A disagreement about whose turn it is to do the dishes picks up last month's missed call and a comment from three years ago, and now there's no single thing either person can fix.

Cortes and Wilson's paper uses a related term, "kitchen thinking," for old hurts coming to mind during a new conflict, whether or not anyone says them out loud.

Because they feel like the injured party, and feeling injured makes hitting back feel justified. Family therapist Terry Real calls this "offending from the victim position," a phrase he credits to an expert on abuse.

The old hurt may be completely real. The trouble is where it gets spent: in a fight about something else, where it can't get a proper hearing and the current problem doesn't get solved either.

1. Acknowledge the old thing. Don't call it old news or argue about whether it counts. Say you can tell it still hurts.
2. Ask to finish the current conversation first. One topic at a time gives each one a chance of ending.
3. Say when you'll come back to it, then do. A "later" that never comes teaches them the only way to be heard is to raise it mid-fight.

Then the old hurt needs its own conversation, on purpose, at a calm time. Keeping quiet about it isn't enough on its own: Cortes and Wilson found that just thinking about old hurts during a new conflict predicted poor relationship outcomes about as strongly as raising them. Name the thing that still hurts and ask for a time to talk about it. If what it needs is an apology, The anatomy of an apology covers what makes one work.

Related: Red flags you're missing: 7 signs a conversation is about to go off the rails