Most everyone remembers where they were and what they were doing on the day of the Sept. 11 terrorist attacks.

Certain sounds, sights and even smells may trigger the memories of that tragic day.

At the same time, many will say the events of their wedding day or the birth of their first child seem like a blur.

In fact, sometimes it seems like the worst memories are the ones that are the hardest to forget. And researchers are just now starting to understand why.

A recent study finds that whether an event is pleasurable or aversive seems to be a critical determinant of the accuracy with which the event is remembered, with negative events being remembered in greater detail than positive ones.

Click here to watch the video, "Why We'll Never Forget"

This is because of an increase in brain activity during negative events, according to Boston College psychologist Elizabeth Kensinger and colleagues, who reported the findings of their study in the August issue of Current Directions in Psychological Science.

Functional Magnetic Resonance Imaging (MRI) studies have shown increased cellular activity in emotion-processing regions at the time that a negative event is experienced, the authors wrote in their study.

The research shows, for example, that after seeing a man on a street holding a gun, people remember the gun vividly, but they forget the details of the street. The more activity in the orbitofrontal cortex and the amygdala, two emotion-processing regions of the brain, the more likely an individual is to remember details intrinsically linked to the emotional aspect of the event, such as the exact appearance of the gun.

Kensinger said that recognizing the effects of negative emotion on memory for detail may, at some point, “save our lives by guiding our actions and allowing us to plan for similar future occurrences.”

“These benefits make sense within an evolutionary framework,” wrote Kensinger. “It is logical that attention would be focused on potentially threatening information.”

A Piece of History

Another reason that people tend to remember tragic events, such as 9/11 and the Challenger crash, is because the memories become part of the nation’s collective history, said Dr. Cynthia Green, a clinical psychologist and assistant professor at the Mount Sinai School of Medicine in New York.

“It’s part of who we are as a people, and it becomes part of who we are as a culture,” she said. “And it becomes something that we personally experience and so we become witnesses to that history, so that we can recount those stories to our children and grandchildren, and talk to them about what it was like for us on that day.”

Even though reliving or remembering tragedies can be painful, it also can be healing.

“I think it’s very hard to say why we have a need to look back and relive these,” said Green. “I think that there is something very comforting in revisiting those memories and visiting them as a group and having the company of others in our generation (to relive and revisit) those memories (with).”

So why do we tend to forget the good times?

“Sometimes when happy things happen to us, such as the birth of a child or a wedding or another kind of special occasion, we’re so anxious and so caught up in that moment that we lose the details,” Green said. “And, afterwards, we feel that we just missed the whole thing.”

Green recommended stopping and focusing on the events or the people present at the events with us.

“Try to really pay attention to the details,” she said. “Try to really get the smell in that moment when you first get your newborn baby in your arms. Take a really good sniff of what that baby smells like and try to really remember what that baby looks like.”