I think another thing that really appeals to me about Bolin and Opal the older I get is that while Bolin in the first two seasons tends to be incredibly flighty, that kid is a lover boy at heart and has so many feelings he can't put anywhere. It's why he cries so easily and falls so fast.
Like from episode 2 we already know he's kind of a chronic womanizer in his own way. He takes girls where he shouldn't so regularly that mako is initially dismissive and deeply annoyed that korra is there. Bolin doesn't have deep relationships, which makes sense because he's a teenager who acts on immediate attraction rather than someone who focuses on the emotional end of things. He saves korra because he thinks she's pretty. He sees a cool, attractive girl being confronted by Toza and cozies up to her.
He gets a shallow crush on korra, yes their friendship adds a layer to the feelings but ultimately they barely know each other and he's able to move past his crush fast because it wasn't that serious. And even going into season 2 we continue this trend of him going off his eyes not the depth of the girls he sees.
He thinks Eska and Desna are hot immediately and which one is the girl doesn't matter he just wants to date her. And then he finds out she's aggressive and abusive and he doesn't know how to break things off. He immediately falls for Ginger who has 0 interest in him initially but, much like with korra, she's pretty and he pushes a bit until he does something that wows her by saving the president. And then it's just a purely physical relationship where any time we see them they were kissing.
Opal is fun to me because it didn't follow his typical route at all. They have fun together and mesh well, it's opal who has the immediate crush on Bolin but she's not pushing for anything. He's perceptive enough to catch that she's into him but it's mako mentioning the idea that spurs him into putting on his Bolin moves. And it fails. Because Opal doesn't like his intense flirting and weird frat boy energy, she likes when he's earnest and vulnerable.
So we get Bolin, loverboy extreme, with his first serious girlfriend because she matches his excitement for the relationship and is just very relaxed and sweet. She challenges him when he needs it and expects him to improve so their relationship can be strong. And because of her pressuring him it means he works hard to be better.
It could've been so easy for the writers to just throw him back with Eska, or to make ginger a reoccurring romance where Bolin just begs and pines. But instead they opted to introduce a girl who matches him happily but isn't beholden to being his girlfriend only. Opal is one of the most stubborn airbenders we ever meet, which matches her easy breezy earthbending boyfriend comedically well.
