fair warning this is honestly a super special reply (dog with hat and lollipop image)

Hey! Great question. Let me open your eyes (probably not really but I'll still explain it)

I WILL first and foremost say that they fall into a very specific ship type that i've liked since i was like twelve lol

Dia's a neurotic perfectionist, and Riko is ALSO a neurotic perfectionist, but in a different way. Dia's a girl from a traditional family, with high and heavy expectations on her shoulders from her family, while Riko clearly has set some expectations for herself (and doesn't believe she can reach them) I think it's clear that throughout all iterations of her character that Riko suffers from some type of depression (Dramas, SID, Pre-Anime, and even in the anime too even tho all of aqours kind of got pushed to be more moe and cute). (Anime-canon isn't typically how I view them canonically if that makes sense)

Anyways, they both kind of play into eachother's caretakers for me. Dia's able to help Riko with her confidence and performance, since she knows how to deal with it (from the pressure of her family, not really something she feels), Riko's able to help Dia calm down and not overwork herself. That sort of dynamic. I genuinely think that DiaRiko is the only Riko pairing in which Riko would NOT lose her mind or get overstimulated. Same with Dia. Idk how to say it other than "they just get eachother and would like being around eachother for more than two hours at a time"

There's a specific radio drama I'm thinking of, it's Dia, Riko and Ruby. Dia's basically telling Ruby to kill herself (as god intended) while praising Riko. (oddly enough she talks about riko being her little sister a lot. it is really funny to me.) She does this a lot in the series. Like in the World Cat Day UR Pair, where she tells Riko she'd be an adorable little white cat. She's super straightforward (sometimes without even realizing it), so I think she could make Riko's heart stop like that... (also going on what i said above... helping riko out...)

various other things:

They're both also weirdos. Riko's a secret pervert, Dia's got the best moe gap in the series...

Out of all of Aqours, Dia's can be BOTH pretty and feminine also very very handsome. Riko's into that given her collection. (CONVERSATION IS ABOUT DIARIKO NOT LOVE LIVE LESBIAN DISCOURSE DO NOT DERAIL) Dia is perfect for the Chin In Hand Top.

Designs compliment each other very well. Voices go excellent together. Have the best UR pairs. Versatile. idk man. they're just great