When Asagai arrives at the apartment, how does his mood contrast with Walter's and Beneatha's?

The most obvious contrast between Asagai and not only Walter and Beneatha but the entire Younger family is that Asagai is, at least outwardly, more optimistic and positive. Because he's African-born, Asagai hasn't had the experiences of African Americans and has a different outlook on life. The description he gives Beneatha, "One for whom bread is not enough," attests not only to his insight but to his ability to see qualities in Beneatha that her own family has not noticed or does not wish to acknowledge.
Beneatha is a young woman in search of something. She understandably doesn't want her life to be consumed by the life-draining poverty and worry that has afflicted her mother and brother. Asagai, unlike her family, understands this, but his attitude, though wholly sympathetic, is partly one of good-natured amusement. Beneatha's emotionalism affects him one way and affects her brother and her sister-in-law in another. One of the most poignant moments in the play occurs when Beneatha declares, "It's man who makes miracles!" and her mother then slaps her and forces her to repeat, "In my mother's house there is still God." Beneatha realizes at this point that her family and its traditional values regarding religion and "accepting one's place" will destroy her if she does not break free. Asagai, in his seemingly untroubled worldview, offers her hope and salvation. In A Raisin in the Sun Hansberry was ahead of her time in anticipating the women's movement and linking it to the Civil Rights movement which at that time was just beginning to gain momentum and success.