It breaks my head.
cousinjean's "The Butterfly Effect" posits that Dawn wouldn't be there in S2, and that makes sense to me. I mean, I could buy either theory. But sometimes I think it'd be fictionally interesting to write early stories as if Dawn were just a normal part of the landscape. You don't see that as frequently though. It's like: here's pre-Dawn Buffy, and she's motivated a certain way and has certain issues, and here's Buffy after Dawn's arrival, and her issues and her *memories* are entirely different. And if her memories are different, *how* are they different? How is she different now as a person? Shouldn't having a sister reshape your personality in key ways? But we never see the implications of that played out in canon; the show never explored how Buffy might have *believed herself* to have changed, given that she has a new set of memories.
I'm not saying this well...what I mean is, the show never talked slyly to us, the audience, to point out discrepancies that *we* would know existed even though the characters themselves didn't. Like, we never got S5 or S6 Buffy saying, "You always make it harder for me to do my job, like that time you came to parent-teacher night and that vamp grabbed you and Spike nearly caught me..." I've seen a few authors do that in fan-fiction, but the show played it safe and didn't beg the question too much.
As if I don't have my own existential crises to worry about. I have to go looking for them....