I’m working my way through Division One, Chapter Six (Care as the Being of Dasein) of Being and Time right now. The analysis of anxiety and its distinction from fear is as straightforward as Heidegger gets, but I’m not sure I understand how it exactly fits into the argument of the chapter.
More precisely, I understand how it’s supposed to fit: we need some way of grasping Dasein “as a whole,” and anxiety as a “basic state-of-mind” has the special property of revealing Dasein’s being-in-the-world as such. If we didn’t have a direct revelation of the wholeness of Dasein’s being-in-the-world, then the analysis of “care” would be in danger of simply adding up the properties we have previously noted, etc., which would be a violation of Heidegger’s methodological commitments.
Yet the transition between the analysis of anxiety and that of care feels forced to me. It’s as though all anxiety is doing for us here is showing the bare fact that Dasein’s being-in-the-world is a unified phenomenon that Dasein can confront as such — and lo and behold, that phenomenon turns out to be unified under the heading of “care”! I understand what the analysis of anxiety is setting up in terms of Division Two, but “locally” it doesn’t seem to fit as well as the analysis of fear (which similarly worked to set up later analyses) did.
(One possible articulation between the two analyses would be if care was to anxiety as understanding is to state-of-mind — but that doesn’t seem to be what’s going on here.)
An und für sich 
We might also discuss Heidegger’s “pre-ontological” evidence for the primacy of care — i.e., the weird myth he (apparently) just stumbled across in some journal article, the random quote from Seneca, etc. There’s a lot that’s strange and awkward in this chapter.
I remember this striking everyone as weird when we got to in John Haugeland’s “Being and Time” seminar. IIRC, the consensus was that most of the weird parts (the poem, the Seneca quote) are purely illustrative, and it’s just a coincidence that this is the only part of the book where Heidegger happens to have a lot of nice illustrations for his points. But I think you’re right that the argument of the book just gets interrupted for a bit while he shows this stuff off. (I also remember we spent a fair bit of time hemming and hawing over why *this* stuff about anxiety was in division one and not two, but I don’t remember the way that got resolved.)
I forget exactly how SuZ was composed: weren’t the latter parts of division one composed hastily? I know that the earlier parts of division one are a lot more polished and clearly-presented, and that he was sending chunks of the book to his publisher (editor?) as they requested he write more before they’d have a complete “thing” for publication. I’m sure that someone else reading this is more familiar with the history than I am; I only know it from reading Rudiger Safranski’s biography several years back.
I think the final sections of division one are a little odd, too, but for a different reason: all of the stuff about Reality and Truth feels to me like it could’ve been an independently-standing essay or short book, and I’m not sure why it’s in SuZ. It’s the best stuff in the whole book, if you ask me, but it feels incidental to the architectonic of the work.
Right, the stated rationale is that he wants to show that his analysis of everyday Dasein has broader ontological payoff — but I’m not sure that it’s really grounded in everyday Dasein, because I’m not sure that the stuff about anxiety and care actually fits within the frame of division one.