Why does Rin hate Sae so much

I haven’t read the manga and just watched S2 Episode 9 and I don’t understand their issues ngl. Like I thought Rin was gonna reveal he got his ass beat as a kid by his brother or like emotionally abused or something. Or maybe I’m just very oblivious to “the deeper meaning” of their relationship.

Like who tf cares if Sae doesn’t wanna be the #1 striker anymore, why is it that deep???