Simply put, whenever a character dies or is killed off, usually they need to be replaced. There is rarely a gap. Either a mentor is replaced by a comforting friend, or a family member is replaced by a lover, or a future disciple. While the replacing character doesn’t usually take on the role of the previous person, they are intended to fill the void. The purpose of this character is usually either a replacement sage, a new direction for the main character’s feelings, or just to keep the number of main characters up in some cases.
You especially see this in people who suffer from the Obi Wan Kenobi complex (See Character Complexes):
Obi Wan Kenobi (DUH!) – You could make the case that he is replaced in Episode 4 either by Princess Leia or Han Solo.
In the Terminator series, Sarah Connor would be rather egregiously replaced by the red-head who plays John’s wife.
… Pretty much every Rocky Mentor: Mickey> Apollo> Duke, and then Rocky’s voice of reason was replaced Adrian> Robert.
In LOTR Gandalf the White replaces Gandalf the Gray… I know, I know, same person, but there are differences between the two characters. Read the book for more info on this.
In the Lion King Mufasa is replaced by Pumba and Timone.
In Bambi, his mother is replaced by… well his father.
Heck even I unintentionally did it a lot in the story I’ve been working on… a couple times in fact! Father Antonelli> Nicolas/Gregory> Federico> Adaline.
Now are their stories out there which do not do this? Sure, most horror and thrillers just kill people off left and right and usually do not refill the empty slot. On occasion you may see a best friend replaced with a love interest or something of that nature, but it’s rare. It also seems like that besides the horror movie exception, few movies can get away from this plot contrivance without losing a lot along the way. Few movies actually come to mind that were any good after killing off a main cast member and not replacing him/her.
So what is it about our stories and our movies that makes such a plot contrivance necessary? Why do we feel the need to build up a character, make others feel strongly (either like or hate) and then kill them off simply to replace them with someone else in the name of furthering the plot? I’m not complaining by any stretch as, obviously, I am admittedly guilty of the same thing, but I don’t know why it’s an almost constantly recurring device.