Though I said this on a thread lower down, James is as typically immature a young man as his sister is an immature young woman. James' immaturity is manifesting itself in being "in love" with pretty Isabella. But I think he'll grow up in the course of the novel, too.

As I also said on a thread below, I think Mr. Allen's marriage to Mrs. Allen must have come about in much the same way as James' and Isabella's engagement. The narrator tells us it is a wonder how any male could possibly like Mrs. Allen. Well, she must have been young and pretty once. And Isabella is an even more grotesque character.