It's usually something that brings up very positive emotions, like childhood memories, wedding/honeymoon pictures, me at various stages of life, important transition periods, people I want to remember well, etc.

For the "negative", I've mostly resorted to journaling in ways that help me self-discover and resolve the past, such as SelfAuthoring.com exercises that I continually write and re-write from time to time (I bought and copied the exercises to my local drive).