I think that it’s more tagged to negative emotions. There are both positive emotions associated and negative but I think the negative ones are more prevalent in our memory. Seems to me it related to the fight or flight mechanism built into our bodies; negative emotions can trigger these types of responses. Think of 9/11, most people in their late 20’s and older know where they were when 9/11 happened, know what was happening. I remember exactly where I was, what I was doing, my dad yelling that he needed the TV.
There are great positive emotions that I remember as well of course but I remember fewer of these as vividly as the negative ones.