tendou saying goodbye to volleyball was like a child saying goodbye to their security blanket. it was always a place of escape for him, where he could unapologetically be his weird self so long as he won- despite how his teams felt about him. but he joins shiratorizawa and finds a group of people that truly let him feel comfortable being his authentic self no matter what. real friends. and so when he says "goodbye my paradise" it's not cause he needs to quit volleyball. it's cause he no longer needs to rely on it as his main source of comfort cause he's got true friends now, willing to embrace every bit of him.