Holden and Sally argue because Holden proposes that the two get married and run away together.  The scene reveals that Holden is desperately lonely and in need of companionship.  He is trying to reach out to someone, but his desperation causes him to be unrealistic and overbearing.  Sally, on the other hand, appears to be relatively stable, and so she does not need the type of absolute companionship that Holden is proposing.  She is happy to simply go on dates and maybe engage in a short term commitment.  The two have vastly different needs and priorities, but of course, neither states directly what he/she needs and wants.