The word “love” appears more than 60 times in the text for Sarah Ruhl’s new play tracing her intense relationship, while teaching an undergraduate class at Yale, with a student named Max Ritvo. Don’t get the wrong idea: Ms. Ruhl was a happily married woman when she met Ritvo, who died in 2016 after a long battle with cancer, and their bond was not a romantic one — at least not in the sense that word is generally applied to couples.