Physics: What is the past? Why does the past exist?

The past is all events in your past light cone. There is no universal past. Every observer has a different set of events in his past light cone. 

The past is stuff you can remember. If you couldn't remember anything you would not survive to ask that question.

Before you read this - that was the past. Read it again - that was the past. Once more - that was the past.

past is when a person or object in motion surpasses the coordinate location of another person, place, or object. it exists in a transitory capacity to denote that an event of motion has occurred