The best answer is "when you are making an observation." While writing an autobiography, you are describing events in your past. If you are observing something, you must distinguish these insights and conclusions from the timeline of events you have been describing. The simplest comparison is to imagine you are watching a movie, the story is progressing, and then the narrator stops, looks at the camera, and says, in the present, his or her observation about what is happening. These observations are happening at a different time from the events of the narrative and therefore are described in the present.
