Why Do We Get Invested?

So...in an hour and a thirty-seven minutes, the greatest show on television comes to an end. (Please don't contest this point for a few months - I'm getting a bit choked up right now.)
After nine seasons, How I Met Your Mother, starring Josh Radnor, Cobie Smulders, Neil Patrick Harris, Jason Segel and Alison Hannigan, is finally ending. And...well, I'm not sure if I'm coping very well. I've been watching this show for a full five years, and as much as I want to see the conclusion, I don't want the show to end. First and foremost, I don't know what I'm going to do with my Monday nights from here on out. Secondly and almost as importantly, I don't know how the show is going to end. Hopefully, it will conclude with, as the title suggests, the main character meeting his future wife. But the writers have thrown us curveballs before. For example, Segel's character's father, a lovable secondary character, dies unexpectedly during the sixth season (https://www.youtube.com/watch?v=o-8Dy_Q4zkE&feature=youtu.be), and NPH's character has some pretty heart-wrenching scenes with his own father, who was absent for his whole life (https://www.youtube.com/watch?v=fmeMbelaZGk). If you don't know the show...this is a sitcom. And there is evidence to suggest that the titular Mother may in fact die before the series officially ends. The theory is that Radnor's character, Ted, is telling his children the story of how he met their mother (and leaves in a lot of details you wouldn't normally tell your kids) because the mother is dead or dying. We'll see.
So, why do I care so much? Why does anybody actually care about fictional characters? Well, I have my own theory about that, though it's probably most of the world's theory as well: I think we care about fictional characters because we relate to them, consciously or subconsciously. When you first see a movie or a T.V. show, there's always at least one character you can relate to. In the case of How I Met Your Mother, I can relate to four of the five main characters. After a while, you get emotionally attached to these characters. And after five years, they've practically become my friends. Watching Radnor's character meet the mother is going to be like watching a friend get married for me. And if the mother (played by Broadway actress Cristin Milioti and introduced to the audience last year) dies, it'll be as though you lost one of your friends. God forbid, if anything should happen to one of the main characters...well, I'll be inconsolable, that's for sure.
Well...the countdown continues. Wish me luck out there, and best of luck to those of you who will be watching. Hopefully I'll/we'll be in a position to continue my/our homework by 10 P.M. tonight.