I've re-read the series a few times now, and honestly, the romantic elements are so subtle they're practically non-existent. You get moments, sure—like Kageyama and Hinata bickering like an old married couple, or Tanaka's over-the-top admiration for Kiyoko. But these are played entirely for comedy or team dynamics. It's a story about volleyball obsession, pure and simple. If someone picks up 'Haikyuu' hoping for any kind of traditional romance arc, they're setting themselves up for disappointment.
That said, the relationships between teammates are incredibly deep and carry a different kind of emotional weight. The trust between Kageyama and Hinata as they develop the quick attack, the way the third-years support the underclassmen—it's all about platonic love and shared passion. The series argues that this kind of bond is just as, if not more, meaningful than romantic love. So in a way, you do get a lot of 'heart', just not the shipping kind.
I think the fandom fills in the blanks massively. You'll find a mountain of fanfiction and art exploring romantic possibilities between various characters. The source material gives you these intense, focused character interactions, and it's natural for readers to extrapolate. But canonically, Karasuno is married to volleyball.