The change reflected is a confrontation of the difficulties that lie between them in Walter and Ruth's relationship.
Walter, in his reluctant embrace of white American society's standards of manhood and success, is quite chauvinistic toward women. This attitude also explains much of the underlying tension and rancor between him and his sister, Beneatha.
However, when Walter comes home drunk, he and Ruth slowly begin to deal with what comes between them.
Initially, he is hostile toward his wife. In a passive-aggressive statement about black people, he blames her for getting pregnant and, thus, making their economic difficulties more difficult. When she tries to offer him some coffee to help revivify him, he rudely refuses. When she tries to offer him something to eat, he complains about how she is always trying to feed him. Her feminine support annoys him. In frustration, she helplessly says, "What else can I give you, Walter Lee Younger?"
In the play's explanatory notes, we learn that "a new mood" has emerged in her, called up after he asks her, "Who cares about you?" He expresses a blatant disregard for his wife whom, he fails to realize, has suffered just as much, if not more than he, from racism and poverty.
Walter admits that things have been "rough," that not much is understood between them, and wonders how people get to the place "where we scared to talk softness to each other." When people are married, but feel trapped by social and economic circumstances, sometimes there is no one to blame, no one to be angry at, other than each other. This, arguably, is the "something" that "done come down between us," according to Walter.
Ruth recollects about how things were when their son, Travis, was born. They were more hopeful, particularly about living in a nice home. Now, "it's all starting to slip away from us." At this admission, Walter turns her toward him and they kiss passionately. In that kiss, there is a sense of resolution: they have very little, but without each other, there may be nothing.
In many ways, "A Raisin in the Sun" expresses Lorraine Hansberry's affirmation of the black family, and its preservation, as a bedrock of community and future progress. Walter and Ruth will remain together because they love each other, and because Hansberry has created two characters who will not succumb to white supremacy's hopes of destroying another black family.