Holden Caulfield is worried that Stradlater, who has a blind date with Jane, may take advantage of Jane Gallagher, who is a fragile and innocent girl.
I kept thinking about Jane, and about Stradlater having a date with her and all. It made me so nervous I nearly went crazy.
Ward Stradlater, Holden's roommate, is a "sexy guy" who takes advantage of girls and he does not care anything about hurting someone. Because Holden knows Jane, he is worried about what can happen to this innocent girl. In addition, Holden has a certain resentment toward his roommate who is handsome, egotistical, athletic and accomplished with manipulating girls.
Holden's anxiety and concern leads to his physical and verbal attack upon Stradlater when he returns from his date with Jane and will tell Holden no more about his date than that they sat in the car all evening. Hearing this, Holden becomes disturbed because he knows that in the past Stradlater has "scored" with girls in cars. So, he antagonizes Stradlater until they fight and Holden is beaten. Having torn up the composition he wrote for his roommate and now defeated by Stradlater, Holden feels even more dispirited at another incident of how someone uses someone else and further proof of the insincerity of people and their exploitation of others. This incident helps to precipitate Holden's departure from Pencey in order to roam New York City as he feels alienated and disgusted with all the "phoniness" of people.