In the final confrontation, Ennis tries to appease Jack by bringing up Don Roe's cabin, (I may off on the spelling), and how they had a good time that year, and he'll try to get the cabin again. Ennis wanted to be with Jack, if only Jack understand that he had to change the date.
Ennis had no idea about, "what a girl wants, what a girl need." When Cassie tells Ennis "that girl don't fall in love with fun" , she was telling him, there was something more about him than fun, and she wanted more from him, she fell in love with him. Ennis just didn't get it.
IMO