In That Was Then, This Is Now by S. E. Hinton, what created a strain in the formally friendly relationship between Mark and Cathy?

In Chapter 6, Bryon, Cathy, Mark, and M&M go for a drive down the Ribbon and hang out together. While they are sitting at a red light, a person in the car to the left of them makes an obscene gesture and a rude comment. Mark then reaches across M&M, opens the door, runs over to the car, and punches the person in the face. After Mark had hopped back into the car, Bryon drives off. Cathy then makes a sarcastic comment by saying, "You're a fast worker" (Hinton 97). Mark responds by saying that he gets these "impulses," to which Cathy dryly says, "Impulses to jump out of cars and hit people?" (Hinton 97). Mark intelligently replies by saying "They hit us first...A hit don't have to be physical. I couldn't hit them the way they hit us without hitting you, too" (Hinton 97). Bryon then mentions how surprised Cathy was at Mark's thoughtful answer. Cathy then comments that she never knows what to make of Mark, and Mark essentially tells her to lighten up and stop trying to analyze people.

Later on, they stop at a drive-in to grab something to eat, and Bryon begins to tell the story to Cathy about the time he was beaten up for being drunk. When Cathy tells Bryon that he shouldn't have been drinking, Bryon says, "I never thought of it that way" (Hinton 99). Mark then enters the conversation and says, "Who would?" (Hinton 99). Bryon mentions

"I realized right then that whatever chance Mark and Cathy had ever had of becoming friends was gone. I had already sensed in Cathy the same hostility toward Mark that he had for her" (Hinton 99).

Bryon is constantly spending time with Cathy instead of Mark, which initially creates a strain in their relationship. Mark had harbored feelings of jealousy for Cathy because she spent so much time with Bryon, but when Mark and Cathy hang out, his true personality conflicts with hers. Cathy is thoughtful and sensitive, while Mark is rash and violent. Cathy does not like Mark's attitude, and Mark cannot stand being judged. After Cathy realizes that Mark does not understand right from wrong, she begins to view him with contempt.