It isn't a new observation that Buffy often gets described as selfish by characters in the show and by fans. "Selfish", "self-absorbed", "self-centered", etc. I kinda find this interesting in a meta-y way. So let me go at it.
This isn't in response to anything in particular, but more to general concepts.
I find it interesting because one of the central themes of the series is that Buffy can't be selfish. As the Slayer, she is required to be self-sacrificing to an extreme degree. This culminates, of course, in S5's The Gift where she actually sacrifices her own life.
Cordelia's purpose in BtVS was initially to portray what Buffy would be like if she were not the Slayer. Selfish. Self-absorbed. Self-centered.
Thing is, Buffy can't be any of these things. She's not allowed to be. In fact, being the Slayer swings things too far in the other direction, requiring an extreme amount of selflessness on Buffy's part - often to an unhealthy degree.
In Prophecy Girl, when Buffy discovers that her duty requires her death, she "selfishly" rails against it. "I'm sixteen. I don't wanna die."
When she sees the distress that Willow is enduring, though, she does it. She goes to her death. Because being the Slayer means putting your own life aside.
We'll see this time and time again, notably in Becoming and The Gift.
This isn't to say that Buffy's never selfish. She absolutely is at times. The most significant being, as recently discussed, her leaving at the end of S2. Also, her depressed funk in S6 is highly selfish. However, I think focusing on that to exclusion of all else is a mistake as it overlooks the fact that those periods of selfishness both come after an enormous trauma endured by Buffy as a result of a selfless act (killing Angel to save the world in Becoming and killing herself to save the world (and Dawn) in The Gift).
Is it any wonder that the demands of being the Slayer occasionally result in a period of withdrawal to cope? Can that be described as "selfish" or is it more of a damage control, so to speak? A coping mechanism to deal with the strenuous demands of her duty?
Not the best coping mechanism, to be sure, but Slayers typically die young for a reason. It's not the type of lifestyle that lends itself to the long term.
I find it interesting how this Selfish-Selfless dichotomy plays out in the series, and in fannish perceptions. In S5, Buffy is determined to not lose Dawn. Take this moment from The Gift:
Buffy: I sacrificed Angel to save the world. I loved him so much. But I knew ... what was right. I don't have that any more. I don't understand. I don't know how to live in this world if these are the choices. If everything just gets stripped away. I don't see the point. I just wish that... (tearfully) I just wish my mom was here.
She gets up, walks a few paces away, turns to face Giles.
Buffy: The spirit guide told me ... that death is my gift. Guess that means a Slayer really is just a killer after all.
Giles: I think you're wrong about that.
Buffy: It doesn't matter. If Dawn dies, I'm done with it. I'm quitting.
Is this a selfish mindset? That Buffy should want to hold on to Dawn after losing everything else, even to the point of later threatening to kill her friends if they go near Dawn?
Likewise, is it selfish to want to get away and spend some time alone after having had to kill her lover in Becoming? Becoming didn't just strip away Angel, after all. It also stripped away her school (which had been integral to Buffy's attempt to remain a normal girl), her home, her fellow Slayer - Kendra. And yet, Buffy did it. Buffy did it, and it damaged her to the point where in S7's Selfless, she's still getting teary-eyed bringing it up. She's still reeling from the emotional trauma.
Her sacrifice in Becoming (and earlier in Prophecy Girl) set the stage for one of the main conflicts of Buffy's character. Being the Slayer means she's "just a killer" because she isn't allowed to be anything else. Everything "just gets stripped away". Not of its own accord, necessarily, but because she's the Slayer. She's often compelled to let things get stripped away. She's required to be selfless to the point of self-destruction.
There's a cultural imperative in society for women to be self-sacrificing. For them to eschew their own feelings in favor of taking care of others'. This is the Woman as Nurturer or Emotional Care-taker role that is frightfully persistent to the point where women often feel guilty taking time for themselves, above and beyond any guilt men might feel for the same activities. Furthermore, women are typically judged more harshly for self-indulging than men are.
That's the context Buffy's in. The Slayer is the epitome of the self-sacrificing woman. The Gift is the apotheosis of this concept. Buffy setting everything to the side, including her life, to save the world.
Of course, The Gift is complicated by the suicidal undertones, which suggests that the selflessness required of the Slayer is so damaging that it often intermingles with the complete selfishness of suicide. Two oppositional concepts leading to the same place. Feeling the desolation that comes from being not only the emotional isolation that comes from being the Slayer but also the responsibility of being the physical protector, Buffy becomes selfless to the point of seeking out purely selfish means to escape. An extreme act of selfishness to match the extreme expectation of selflessness she endures.
This puts Chosen in an interesting light. The "solution" the show presents to this unrealistic expectation for women is to share not only power, but responsibility. Buffy's burden is lifted because other girls now share the Slayer responsibility. When the duty is not resting all on one shoulder, it becomes more bearable as it's not all down to one girl in all the world.
I'm not sure exactly how to translate this out of the metaphor in this framework. It could be seen as an encouragement to form friendships and bonds with other woman - share the emotional burden. This may seem a bit obvious, but given how much encouragement women receive to compete with each other, it's actually nicely progressive. I think there's something more there that I'm just not able to work out. I'll throw it out there for anyone else who's interested.