I love this scene. Willow has many reasons to be upset. Yes, she got back together with Oz. However, that doesn't change the fact that she still has lingering feelings for Xander. Willow loved Xander her whole life. She planned hypothetical weddings with him as the groom.
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Second, it's not just about jealousy. Xander said in Becoming that he and Willow would call each other every day and talked about that they did all day. Willow thought she knew Xander and she thought that he was growing up at the same time as her. However, apparently not. Xander had this huge life event and didn't tell her until he made a public declaration at the Save Faith meeting. Oh, btw the meeting was about how Faith killed a man and tried to frame Buffy and isn't taking responsibility and it came out that Faith treats her stallions like they mean nothing- so Willow also has the trouble of worrying that Xander lost his virginity to a possibly destructive and violent girl who doesn't even care about Xander.
"Selfish" shouldn't even be a part of this conversation. Yes, Willow has deep emotions- some of which are healthy and some of which are unhealthy and jealous. However, Willow made a very *unselfish* choice to bury her pain as best as she can in the Scooby meeting and then have a good cry all by herself where she won't trouble others with her feelings. Willow wasn't troubling anyone or being rude or being selfish- she was just feeling her full spectrum of emotions. Now, if only someone developed spells to forcibly limit Willow's emotion spectrum than she could turn herself into the kind of gal that everyone would love. Hey! What if instead of crying, she willed that her heart would be healed! Everyday fandom's reactions to Willow having emotions makes her tragic choices to control those emotions with magic make tons of sense. There really isn't room in this world for the Willows of the world to experience pain (even privately) without fans yelling at her for...feeling pain.
I also admire Willow for saying that she thinks Faith maybe belongs behind bars but she is biased because she has issues with Faith taking her people. Right away, Willow proposed the rehabilitative method that Faith actually needed. As it turns out, Faith needed a time out in prison. Faith had committed prison-worthy acts and her attempted rape, attempted murder, and full on battery of Xander was beyond the pale and should and DID enrage his best friend Willow. However even with all of that, Willow freely disclosed all biases that she may have when she's proposing Faith's treatment. That's the way the world should work- people speak their mind but include their biases. However, as always, since Willow discloses her biases and flaws, fandom castigates her and falls all over themselves with adoring characters who never admit their biases and flaws ala Giles or Angel.
And Willow absolutely had reason to fear losing Buffy to Faith. It was insecurity- but it was based in logic. Buffy feared losing Willow as a friend when Willow stood her up in Dead Man's Party. Well, Buffy stood Willow up at their chemistry study session to hang with Faith. Buffy flat out told Willow that Willow doesn't get her since Willow is not a slayer. Buffy flat out revoked her invitation to Willow to come slay since Willow is not on a slayer- and their friendship is largely built on the slaying. If Willow can no longer routine patrol with Buffy, Willow misses out on a huge part of Buffy's life. Faith has a huge advantage to understanding Buffy and playing her world- especially if Buffy embraces that and shuts out her non-powered friends and Buffy was doing that. If Faith hadn't gone off the rails, I could see Buffy blowing off her non-powered friends more and trying to play in Faith's world for a longer period of time. It didn't seem like Buffy missed Willow like Willow missed Buffy.
