Something in my soul and mind that they are exes either that or Kuroo had a crush on the snake, Daishou, and is coping with the memories of having simped over the man by teasing him
It doesn't help that Daishou's girlfriend, Mika (who is very pretty btw), thinks that she is in a love triangle with Daishou liking her and Kuroo who may or may not like Daishou and Daishou may or may not have had a crush on him. I think that's so funny
Now, these two are either bitter exes and their relationship ended on a bad note and that is why they tease each other. They have that energy of exes who are not over their breakup and are hung up about the bad things they did in that relationship
Either that or Kuroo had a crush on Daishou for a while but when he found out Daishou asked out Mika and that the two were in a relationship, he didn't know how to deal with the heartbreak and instead acted as if everything is fine and is dealing with it by teasing him. He's probably over it but the reminder that he used to have a crush on Daishou is embarrassing enough to continue to tease him as if that temporarily erases the fact that he had a crush on him
Either way, there is a weird tension between them that I can't help but notice

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Haikyuu Headcanons
FanfictionHeadcanons that I believe have happened to the characters off screen that may or may not be factual