Kanaya is bitter when she realizes she can no longer control and manage space, it's gotten too big and she's overwhelmed and Aranea comes into the picture to help lift her of the responsibility of being the primary Sylph in the dream bubble. Aranea needs to regulate with salt due to her Cerulean nature and she keeps losing capacity to retain information unless it's a little bitter. The bitterness reminds her that the people she's mind melding with are still flawed and that putting Kanaya on a pedestal and expecting her to take care of everything when she's overworked is going to doom the timeline. The Sylph of Mind comes in and unburdens the Sylph of Space to make room for the idea that maybe you can't control everything. Maybe you don't have to, and a lot of people get hurt when you make empty promises and ignite hope in people with failure to take meaningful actions. It's easy enough to say that you care and are going to help, but being salty allows you to realize that maybe the people around you aren't always the ones you should be around. Purity and morality OCD are going to destroy people who want to speak out against injustice but struggle with finding a voice. Sometimes the people you are around are the ones that are allowing the rot to set in, and the stagnation furthers the rot. Rose stays around Kanaya because she loves her, and Kanaya loves Rose, supposedly. But to love someone is to have the courage and respect to call out people who actively hurt others. Just because someone says, "Trust me, I'll handle this" does not mean they can or should. Trusting someone who actively ignores the suffering of people who aren't their friends is holding the session back.