The large ensemble shows seem to have it easier. The new characters are welcomed, and even loved. Tara from BtVS was, after all, introduced late in the show, and mainly as Willow's new love interest, but she had become a character in her own right, and was much missed and mourned after her unfortunate demise.
Another show - CSI:NY. Lindsay Monroe, Aiden's replacement. A Latecomer. Recently a Love Interest as well, destroying three slashy ships of mine in one go... And I love her. Not necessarily a popular fandom opinion, but still. She is, however, grudgingly accepted by the fandom. But then again, CSI:NY is an another ensemble show.
Supernatural, however, is centered on two characters and two characters only, and so is its fandom. And to add to this, fandom seems to be bent on slashing the Winchester boys six ways from Sunday. Not that there's anything wrong with that - I love good Wincest myself, but anyone who actually thinks the canon will go this way is as delusional as... I can't think up a suitable simile, sorry. But as long as the only girl appearing is Jess' ghost, or an occasional Chick of the Week, fandom in general is happy with fanfics and manips and vids and the happy and pretty land of slash. And rainbows.
And then She appears, the girl who doesn't want to be boxed in a one-week episode, be saved, grateful, and left behind. A girl that dares to be skilled with a rifle, knows about the supernatural, wants to be a hunter herself, plays poker and (gasp!) seems destined to be Dean's love interest, therefore destroying the perfect picture of Sam and Dean getting it on whenever they can.
I'm exaggerating, I know, but I get a little annoyed with fandom and the general panicked response. I get a little bit more annoyed with the show's creators, for giving in and apparently writing Jo off, first as the prospective love interest, then in general.
The thing for which I love Supernatural the most, is because it follows certain patterns and archetypes, hitting many of my favourite movie/literary/storytelling kinks at the same time. The road tale, small towns and the constant movement. A horror story, with all the cliches and solutions. An epic tale, a hero's story, a journey spurned by the loss of familiar and beloved, to find out who you really are, to conquer the evil and to conquer your own dark side, the journey not only literal, from point A to point B, but also a journey inside.
Same reason I love Star Wars, you know? Same reason I fell in love with Neil Gaiman's American Gods. Same reason I love Tolkien, Arthurian legends, myths and folk tales. The repeated journey of the hero. The monomyth.
Sam is the hero, of course. There's everything there what should be there. The call to the adventure, the herald (Dean), the refusal to follow, the road of trials, atonement with the father... you know this. You've heard this before.
And read my lips: Sam is the hero. He's Arthur. He's Frodo and Aragorn, he's the freakin' Luke Skywalker.
He doesn't really get it on with Han Solo, no matter what some fans seem to think (yes, me included, from time to time, but hey).
And Dean. Dean is Han Solo. You know that, too, it's been pointed out so many times I feel damned repetitive. And Han Solo gets the girl, we want him to, don't we? Damn it, most of us wanted to be the girl, at one point or another.
And Jo, Jo who gets into troubles, but doesn't really need saving, except when she really does. Jo, with her guns and her knives and her quick tongue. Jo, pretending to dislike Dean, even when it's obvious she fancies him. Jo is a part of the story, and damn it, I want her in the story.
And besides. I like cute, smartass blondes who can handle weapons. So gimme Jo, damnit.