ahaha okay so i wasn’t gonna talk about this because as a seiusa shipper this scene hurts my heart and also tends to be a scene the fandom reacts unnecessarily violently towards but i got sick of the void of empathy so here’s my thoughts
bear in mind two things: a) im not very good at analysing stuff and b) i’ll refer to seiya with she/her and i’m not arguing with regards to that (actually, im not arguing with regards to any of this stuff, this is just me attempting to shine a light on seiya’s motivations or perhaps, more accurately, lack of motivations)
for context, this scene comes at the end of an episode in which we are shown that seiya is preparing to move on from her “new life” as it were and this, unsurprisingly, is a difficult and emotional stage for seiya’s character. seiya, unlike yaten and taiki, actually settles really well into life on earth, so much so that she actually falls in love with usagi. this episode is the point at which seiya truly realises the extent to which she has fallen, when she sees usagi rather than kakyuu in her daydream about home.
i think, for a sixteen year old with a huge responsibility who is experiencing a new planet, that’s gonna be confusing as hell. seiya’s entire life and existence and purpose on earth has (supposedly) revolved around finding kakyuu thus far, with everything that came with being an idol just being part of the disguise. here though, we see that there’s more to it than that, and that seiya is indeed emotionally invested in her earth life and more specifically, in usagi. this has always been a part of her character but i think this is where it really comes to the forefront.
this all culminates in the above scene which has led to…pretty disturbing responses calling for seiya’s head which is…yikes, but anyways. it’s important to note that this scene comes after seiya has already prepared to say goodbye, after she sees the drawing of usagi and mamoru on the desk and comments how their time together has been short, but fun - there is no malice, nor motivation of any kind to change usagi’s mind or whatever. that isn’t part of who seiya is as a person and it never was.
whilst seiya frequently displays a casual side to her approach to her and usagi’s relationship, this doesn’t mean that seiya doesn’t have feelings too. and it doesn’t mean that she isn’t hurting as well. this is something i think the fandom at large either fails to pick up on or really doesn’t care about and like, idk, maybe it’s worth some thought.
seiya is upset in this scene. she’s upset and yes maybe she’s jealous, because she is there and she is protecting usagi and she cares so deeply for usagi. and this mamoru person (who seiya knows nothing about by the way - unlike the audience) is absent and possibly, in seiya’s perspective, appears to not care at all and yet usagi can still only think about him. and that’s going to be fucking painful. seiya doesn’t know the extent to which usagi is suffering either, because usagi up until that point has publicly kept it quiet because she is a brave little soul.
and so, seiya slips up. and i cannot say that what she says in “am i not good enough” was the correct thing to say because of course it isn’t. she takes usagi’s pain and suffering and she does make it about her. but here’s the thing - seiya is also suffering at this point. seiya is also going through pain. and seiya reacts based purely on emotion at this instant. unrequited love hurts like hell and i personally can’t begrudge her the opportunity to make a mistake here in how she deals with that. this isn’t a planned line or something she necessarily thought would make usagi change her mind or anything like that. this is literally a teenager in love expressing pain and a lack of self-confidence and not being careful of usagi’s feelings whilst doing so, because she is wrapped up in her own pain - as is natural for her to do, because seiya kou also experiences emotion, and seiya kou also sometimes lets that get to her.
it’s a mistake yes, but it doesn’t make her a bad person. we all make mistakes, especially where emotions are concerned, and i think it’s important to make note of that. im not saying u have to love seiya or even like her, but baying for her blood because she fucked up slightly in an emotional moment is just too much.