Memories  that stick with us are tinged with emotion. 

All memory, as McGaugh explained, is colored with bits of life experiences. When people recall, “they are reconstructing,” he said. “.... It means that they’re telling a story about themselves and they’re integrating things they really do remember in detail, with things that are generally true.” ..... For all of us, the stronger the emotion attached to a moment, the more likely those parts of our brains involved in memory will become activated. As McGaugh told me, you wouldn't remember every commute you took to work each day. But if along one you witnessed a deadly crash, you would likely remember that one. Memories that stick with us are tinged with emotion.

http://www.theatlantic.com/health/archive/2013/11/how-many-of-your-memories-are-fake/281558/