The hippocampus is the "aware" part of the brain, it's a kind of short memory (from 1 second to some months). There, events can be memorised very fast, no repetition needed, to save information. Only important information will be stored here. So, the question now is:

What are important informations?

All sensoric and motoric events, which are related to a changement in the emotional arousal level. The emotional arousal is directly linked to the needs of the organism.
Those needs are:
- fundamental needs: energy, protection, selfreproduction
- informational needs: avoid uncertainty

The hippocampus act as "timeline event linker", it links sensoric and motoric events together over time whitout stop. This results in endless chains of events. But not the whole chain of events will be stored in memory forever, but only the "important" parts. As mentioned above those important parts are determined by the strength of changement of the emotional arousal. So, this results in parts of the event chain to be stored, other parts to be lost. Parts of the event chain in the short time memory (in the hippocampus) also can be overwritten, which results in merged, modified or lost event chains.