The urge to ship same-sex pairings in Haikyuu!! is kind of inevitable due to having a primarily male cast, although it's not as blatant as other sports series with a predominantly male cast.
Notable is that Furudate's habit of introducing characters in duos provides fans with a lot of conveniently non-intersecting, ready-to-ship pairs with unique dynamics. It's also worth noting that every setter except Kageyama is commonly shipped with his captain/ace, and even Kageyama is shipped with his 'special' spiker and not just the team's ace. The spiker/setter trust goes a long way in fanfiction.
- Hinata and Kageyama's Vitriolic Best Buds friendship attracts a certain type of shipper like flies to honey.
- Their relationship grows into one so deep that even other characters often mistakenly think they've known each other much longer than they actually have, such as Ukai asking Takeda if they've played with each other before or are childhood friends.
- There are times where they are referred to as partners. Sugawara even notes to himself that Hinata, as much as he looks up to his upperclassmen, looks much happier spiking Kageyama's tosses compared to his own.
- Yachi is distraught when Hinata and Kageyama get into a bad fight after disagreeing over their quick attack, and wishes nothing more than for them to make up and be friends again. During the week-long Tokyo camp, she keeps an eye on them and goes out of her way to offer to throw the balls to them so she can get them to practise together again.
- At their introduction, Nishinoya and Asahi behave like a couple trying to make up after a big argument. Afterwards, they often shown together and it's clear they look up to and count on each other a lot.
- Oikawa and Iwaizumi are very close Childhood Friends who act Like an Old Married Couple and are rarely ever seen apart.
- Childhood Friends Kuroo and Kenma are a popular pair, largely because Kuroo is the only character besides Hinata that Kenma is comfortable around. The backstory of these two first meeting each other and how they developed their friendship and interest in volleyball together helps a lot.
- Itís almost inevitable to see some of this between Sugawara and Daichi, thanks to their status as Team Mom and Team Dad for Karasuno and Sugawara being Daichiís Number Two. Sugawara is also the only member who can tell Daichi off and get mad at him, as other people have noted.
- Ukai and Takeda are both in their mid to late 20s and Karasuno's two coaches who often talk to each other during matches and training sessions, so they are unsurprisingly often shipped together.
- Takeda is very persistent to get Ukai on the team and doesn't give up on him no matter how many times Ukai turns him down.
- Early on in the show, Ukai yells "Are you trying to seduce me?!" to Takeda. This was his response to Takeda informing him Karasuno will have a practice match with Nekoma (and is what finally catches Ukai's interest), but it can be easily misunderstood as otherwise.
- It's hard no not raise an eyebrow at Akaashi and Bokuto's backstory. Immediately after seeing the later, Akaashi decided the was a "star" in his eyes and followed him to his school, spending a great deal of his high school years taking care of him. They are show to be friends even years later, even having the same nickname for the twins.
- Yamaguchi is the only person that Tsukishima tends to show genuine concern for and Tsukishima also knows best on how to comfort and support him. Tsukishima also can be quite defensive for Yamaguchi's sake. Also, Yamaguchi is the only person allowed to call Tsukishima "Tsuki".
- Yachi and Shimizu are the two female managers of Karasuno and so conveniently get shipped a lot due the many significant interactions they have with each other.
- Yachi only agrees to becoming a manager at first because she was spacing out admiring Shimizu's beauty and not listening to what she's saying. After she first meets Karasuno, she panics over any potential diehard Shimizu fans coming to murder Yachi because of how long she hung around with such a pretty girl.
- Shimizu herself also becomes much more talkative when talking with Yachi than the boys (something some of the team picks up on). She even opens her up to her a few times, sharing personal details about herself that she doesn't share with the boys such as the reason why she always covers up her legs.