On Respecting Canon
What Te Said. I'll wait here while you read it.
Characters, in fiction and in real life, exist within a web of relationships. John loves Jane, condescends to Mark, respects and admires Susan but isn't her friend. Even when you're completely under that first limerant spell, there are other people in your emotional life, people who matter as much as your romantic lead.
Presumably you're writing fanfic because you like the show/movie/band in question. At least, I hope so. In that case, why don't you respect the characters? If you're writing a relationship not found in the show, there's a very good chance that another character is in your way. It is possible to remove that character without announcing that s/he's a selfish bitch who is completely unable to appreciate the ineffable wonderfulness that is your protagonist.
Romantic relationships don't always end because one partner is a blind, sadistic wretch who is unworthy. (Except for mine, that is. All my exes showed appalling bad taste, and will surely be punished at the Dreadful Day of Judgment. But I digress...) People move on because they're tired, because the relationship stops being fulfilling, because they have fallen desperately for somebody else.
Which brings me around to Buffy. Buffy isn't just the show's title character, she's a Hero. Which means, unfortunately for her, that her Duty often has to take priority over her emotional life. But that doesn't make her the selfish bitch who so often shows up in fanfiction. Sure, sometimes Buffy drifts away from her friends; however, the show always portrays this as a serious mistake, not as the inevitable consequence of her self-centered eviltude. Estrangements are followed by admissions of wrong, which are followed by reconciliations.
99 times out of 100, when I see Bitchy!Buffy in fiction, it's because the writer has overidentified with another character. Saint!Willow, or Underappreciated!Xander. How DARE Buffy disapprove just because a close friend of hers is boinking a demon vowed to destroy all she holds dear? Doesn't she know that Willow is her Best Friend, and is kind, sweet, and smart? Similarly, how DARE Drusilla get huffy just because the guy who vowed eternal love to her has fallen hard for the enemy of all his kind? Doesn't she realize that this is Spike we're talking about? The guy with the repartee, the coat, and the cheekbones?
Well, yes, they do. But, if they're real, breathing (sorry, Drusilla) characters, friendship isn't blind. You can love somebody dearly but still believe, for well-founded reasons, that s/he is making a bad mistake. God knows Xander has never entirely forgiven Buffy for loving Angel, and that Willow still resents every non-Willow girlfriend Xander has ever had. Friendships have tensions. That doesn't make them any less loving. It makes them real.