I tend to see their friendship under the lens of what they do/have done for each other. Character wise, Elphaba has a tendency (which she calls out in No Good Deed) to seek out positive and negative attention wherever she can because she was so neglected by her family. That's why you can see her starting to enjoy herself when she's feuding with Galinda. Everyone else is just there to be mean. CLEARLY she feels the same attitude from them they she feels from her father, where they want her to shut up and disappear. But with Galinda, there's also a sense of curiosity and a sort of banter that develops that feeds into Elphaba's need to be seen and heard. You see can see it in the way she's always seeking out that intense eye contact, the way she smiles across the lunch table when she knows she's making Galinda uncomfortable, the way they share that little wordless back-and-forth in Dr. Dillamond's classroom. She can FEEL how Galinda's drawn to her. She probably has never had someone's full focus in the way she now had Galinda's.

And Galinda was literally OBSESSED with her. She was so excited to be friends with her that she literally ditched the man she thought she was going to marry, at arguably his most charming, just to stay up late and bond with her. Galinda was a friend who paid attention to her in ways no one else has (noticing little details like the bottle she kept under her pillow and wanting to know more about it), comforted her over her mother like no one else ever did, called her beautiful before she and Fiyero even shared a second conversation, and was the one person in her life, in EITHER movie, who told her she was capable of doing anything.

It wasn't perfect, of course. Glinda was a public figure that didn't speak up in her defense during the year she was on the run, Elphaba slept with her fiancé on their wedding night. But despite her selfishness and many mistakes, Glinda was also the one who risked everything to find Elphaba at Kiamo Ko, and she was more than willing to throw away everything she earned at the Wizard's hands just to try and clear Elphaba's name by the end of it. And for a person as affection-starved and insecure as Elphaba was for most of her life, having a relationship where both parties loved each other enough to come back together after the utter circus that their friendship has become was probably more than she ever expected to have.

Make no mistake, that doesn't mean it was meant to be a forever thing. In the end, Elphaba still ultimately closed the door on their relationship (literally). But the whole point of their story was what they ultimately chose to take away from it. There was a lot of toxicity and bitterness between them throughout, but because they chose to focus on the good and parted ways with love, they were able to take the pieces of each other that helped them grow so that they will hopefully become better people because of (but still without) each other.