This important part of the play comes at the end of Act Two Scene 1 and we can clearly see the various hopes of dreams of the characters involved. For Ruth, the news that Mama has bought a house for them all comes as a godsend. Note the way she responds:
HALLELUJAH! AND GOOD-BYE MISERY... I DON'T EVER WANT TO SEE YOUR UGLY FACE AGAIN!
For her, their present abode is a source of oppression for them and is a great stress. It is infested with cockroaches and there is not enough room for them all. Especially given her condition, as she is expecting again, she was despairing of how to bring up another child in such a location but also provide for it. The news of this house gives her hope and a future for both her family and for the unborn baby inside of her.
However, for Walter, the way that Mama has used the money illustrates her headship of the family and represents the death of his dream to start up his own liquor store. Note what he says to Mama in his anger:
What you need me to say you done right for? You the head of this family. You run our lives like you want to. It was your money and you did what you wanted with it. So what you need me to say it was all right for? So you butchered up a dream of mine--you--who always talking 'bout your children's dreams.
We can see from this quote that, although the new house represented a hope and future to Ruth, to Walter it represents the way that his dreams and hopes have been squashed. Mama has acted as the head of the household, which has slighted Walter as the eldest male of his family.
Ruth is thrilled that Mama has bought a house. She believes it will finally give the family something they can call their own. More concretely, it will allow them to move out of their run-down apartment with cockroaches and rats, and into a place where there will be room for the new baby. Since Ruth is primarily concerned with the welfare of her family, the new house is the fulfillment of her dream.
Walter, on the other hand, is furious. When Mama buys the house, it ensures that he will not be able to spend the insurance money on his liquor store. Since Walter is primarily concerned with making something of himself and becoming a business man, the new house is the death of his dream.
Ruth and Walter have completely opposite reactions to Mama's good news in "A Raisin in the Sun." For Ruth, the news that Mama has purchased a house with her husband's life insurance money is a dream come true. Ruth's motivation for her actions in the play are her family - primarily, her son. The Chicago apartment represents oppression and hopelessness to her as she has to fold up her son's makeshift bedroom each morning and go to work, slaving in someone else's house just to support the Youngers' meager lifestyle. An actual house with separate bedrooms and its own yard represents the end of an era for her.
For Walter, Mama's purchase is the death of his dream. His motivation is also his family, but Walter has a deep internal conflict driven by his inability to provide a way for his family to move out of their current situation. His hope was to take his portion of the life insurance money and invest it in a liquor store with the hope that his investment will give them the lavish lifestyle he covets. When he learns that Mama has bought a house for the family, he feels that he has been robbed of his "American Dream" and his manhood. He is immediately bitter towards Mama for shattering his hopes of fortune.