They can match each others energy! Honestly, besides "the foundation of their dynamic is a strong friendship in which they just get each other" part, both of them putting out similar vibes is a big reason for me. They're both mood makers and supporters among the karasuno team and being together it means they don't have to occupy that position all the time. They're on equal footing in that sense. Also I just really like relationships that are build on good, solid friendships. There is just an established trust between them