In every story, there must be a past. The reason for creating a past is for the hero and mentor to overcome mistakes from the past. Without a past that haunts the hero and/or mentor, the hero and mentor characters risk appearing too thin and one-dimensional.
Think of every great movie and the hero or mentor has a problem form the past that pops up to haunt the hero or mentor later. Sometimes we get to witness this haunted past. In “Brooklyn,” an Irish girl must deal with a bitchy shop keeper who treats the hero (and everyone else) poorly. The hero later goes off to America and gets married. Then she returns back to Ireland and starts hanging around another man. Unfortunately, the bitchy shop keeper has discovered that the hero has gotten married and threatens to tell everyone that the hero is married and hanging around a single man. Now the dilemma for the hero is should she go back to America or stay in Ireland?
In this case, her past comes back to haunt her and complicate her life. In other stories, the haunted past has occurred long before the story even starts. In “The Karate Kid,” the hero’s mentor (an apartment handyman) has lost his wife and son years ago and still feels sorrow for this tragedy. Only later does the hero discover this and realize that the mentor is emotionally hurt. Now when the hero defeats the villain, the hero also helps validate the mentor and make the mentor realize that although his wife and son are never coming back, at least he’s helped someone else triumph.
That’s the purpose of the haunted past. It makes the present actions more emotionally charged. In “The Karate Kid,” the story would be the same if we never knew about the mentor’s haunted past, but knowing this helps make the mentor feel more vulnerable and more like a real person. Then when the hero triumphs, the mentor triumphs as well and that extra emotional pain from the past is at least partially wiped out.
Look at “Terminator 2” where John Connor still loves his mom, but is sad that she’s locked up and thinks she’s kind of a nut. Only after John Connor sees the Terminator for himself does he realize his mom was right all this time. Knowing this helps erase the painful past he had believing his mom was crazy.
What happens when the hero or mentor don’t have a painful past that they must deal with? Then the story risks feeling flat and distant with little emotion behind the story. Without emotion, a story will simply not be interesting, and a dull story is something that nobody wants to see.