Why would they make memories so flammable?

Boscha: It’s so wierd that a human goes here now. Too bad that she’s already burned her social life at the stake. Any friend of Willow’s is an automatic dork.
Hey, don’t go hating on dorks! These dorks are precious.
Skara: Amity, weren’t you friends with Willow?
Amity: I’m a Blight. We only associate with a select few. Keep annoying me, though. I’m happy to select fewer.
Luz senses a mystery, and she sniffs out a few more tantalizing hints when picking Willows brain for memories.
And as with many things in this show, that’s not a figure of speech.
Luz: Hey, I know you and Amity have a history. Wouldn’t you feel better talking about it?
Willow: No.
Luz: If you won’t tell me what she did, I’ll never be able to cook up a scheme to make you friends again.
Willow: Luz, I get it, but I don’t want you to do that. No schemes, no plots, no ruses. None. Okay?
Luz’s intentions are obviously good, but she is prying into territory where Willow has clearly said she doesn’t want her help.
Luz: She never did mention anything about…shenanigans.
And by “shenanigans”, Luz means flipping the memory print of Willow and Amity, which Willow had turned so that she wouldn’t have to see a reminder of their past friendship (and it wouldn’t be visible to random passersby).
Guess who just happens to pass by?
And accidentally sets Willows memories on fire. Well, at least most of them were an accident – she only wanted to burn herself out of the picture.
Of course, Amity’s not heartless, and when she discovers that her little fire essentially made Willow forget EVERYTHING (including a few necessary survival skills), she goes home with Luz to try and fix her screw-up.
Eda: So, you set Willow’s memories on fire and erased everything. Even minor damage can have huge effects […] Yep, her brain’s burned up real good. Damage like that can change everything about a person. Willow may never be the same.
Eda somehow has a spell to take them directly into Willow’s mind (and she’s clearly familiar with mindscapes, at the very least in theory). So it’s all super sketchy, but as this appears to be their only option for fixing Willow, they all go along with it.
Luz: Amity, you have to go with me.
Amity: Willow…might not want me in there.
Luz: You set everything “in there” on fire! You have to help.
Eda: Bold choice, taking the girl who caused the mess.
Well, it was either her or an extremely distracted Gus…
And Amity proves to be surprisingly useful in Willow’s mindspace.
She shared a suspicious amount of important memories with Willow, which helps a lot, as they discover they have to fix some key aspect of the memory.
Amity: Willow was a late bloomer…
Luz: I know. But now she’s more like a great bloomer! This place is like a theater showing her most important moments!
Amity repeatedly brings up the fact that Willow struggled with her magic for quite a while, basically until the start of the series when Luz helped her show what a talent she had for plant magic.
Soon enough, they get the hang of fixing the memories, but Amity’s still pointedly avoiding the memory she first tried to burn…
And Amity also spots this suspicious person wandering around Willow’s mindscape, but Luz doesn’t.
Luz: Amity, you’ve gotta stop being weird. We have to fix all of them. Unless…there’s something in there you don’t want me to see. Look, I’m not here to judge, I’m just here to help Willow-
Amity: Help? All you’re doing is prying into your friends’ lives! Well, did you ever think maybe it’s none of your business?
To be fair, Amity kinda made it her business by frying Willow’s mind. But she still has a point.
Then Luz discovers the flame-person, too, and what’s more, she figures out what it’s doing: It’s only burning the pictures with Amity.
Luz: Amity, that thing isn’t after any photo. It’s after you!
But she also cooks up a plan to defeat it.
Nothing to see here!
But after the flame-person takes a dip in the sea, it becomes more recognizable.
Amity: Are you…the Inner Willow?
Inner Willow: I was. Love, sadness, fear…I used to be made of all emotions. But ever since you set Willow’s mind on fire, all I can feel is anger!
Luz: Please! Stop!
Inner Willow: But Amity wanted this! Every moment she touched I’m going to burn!
Amity: You’re just hurting Willow! Why are you doing this?
Inner Willow: Still haven’t figured it out yet? Then I’ll show you!
Inner Willow: You said I was hurting Willow? I’m just finishing what you started!
So we finally discover what drove a wedge between Willow and Amity.
Young Amity: You have to get out, Willow!
Young Willow: Wait, why? […] Is it because I still can’t do magic? Amity, I’m sorry I got us in trouble at the beach, I just- I can’t get the spells right…
Young Amity: Well, yes, that- that is why. Because you’re a weakling! You can’t do magic, so I don’t want to be your friend. Now go!
Inner Willow: Then you let your new friends pick on her for years, all because you thought she was weak. Well now I can erase all that pain.
But then Amity explains what led up to this scene.
It’s not really clear how Amity can show her memory in Willow’s mindspace, but it’s effective storytelling, so I’ll give it a pass.
Mrs. Blight: What is Willow doing here? She wasn’t on the guest list for a reason.
Young Amity: But she’s my best friend.
Mr. Blight: Nonesense. Blights only associate with the strongest of witchlings. You may choose a new friend from one of the suitable companions we invited.
Young Amity: But they’re mean! Just because you work with their parents doesn’t mean I have to like them!
Mrs. Blight: Good children don’t squabble, dear. Sever your ties with Willow, and if you don’t…
Mr. Blight: Then we will.
Her parents threaten to keep Willow from attending Hexside (somehow), so Amity caved to her parents’ demands.
Amity: Willow, you were never too weak to be my friend. I was too weak to be yours. I can’t take back what I’ve done, but I can promise you this: I won’t let Boscha and her gang pick on you ever again!
Amity finally realized how much she hurt Willow, at least partially due to how many of Willow’s prized memories included her.
Inner Willow: I think Willow should keep her memories of you. The good, and the bad.
Luz: So does this mean you and Willow are buddies again?
Amity: I’m not sure.
This is one thing The Owl House actually does better than Steven Universe: Depicting changes in relationships as a process, as opposed to just flipping a switch. Amity might have had a change of heart, but until she backs up her words with action, Willow has no more reason to trust her than before, and regardless of the reason, Amity still hurt her a lot.
Willow: I remember what you did, too. What you did in there…I can’t say we’re friends, but it’s… a start.
Next time: A marginally lighter-hearted adventure with Amity…