What causes Walter to change? Use quotes to support your answer.

In act 3, Walter Jr. ends up inviting Mr. Lindner over to their apartment in order to sell him back Lena's newly purchased home in Clybourne Park. Walter Jr. believes that he can redeem himself and earn back the lost money by selling his family's home. When Mr. Lindner initially sits down to do business, Ruth tells Travis to leave, but Lena stops Travis and says,

No. Travis, you stay right here. And you make him understand what you doing, Walter Lee. You teach him good. Like Willy Harris taught you. You show where our five generations done come to (Hansberry, 146).

At this moment, Walter Jr. seems to experience an internal change and begins to think about his ancestors. Lena's words motivate Walter Jr. to start thinking outside of himself, and they essentially guilt him into considering the progress his family has made over the generations. Walter Jr. proceeds to gradually elaborate on his family's makeup by mentioning that he comes from generations of plain people. As Walter Jr. speaks, he gains integrity and begins to fully understand the weight of his decision in regards to his family's legacy. After Walter Jr. mentions that his father almost beat a man to death, he says,

Yeah. Well—what I mean is that we come from people who had a lot of pride. I mean—we are very proud people. And that’s my sister over there and she’s going to be a doctor—and we are very proud (Hansberry, 147).

Walter Jr. then makes the honorable, defining decision to reject Mr. Lindner's offer and vows to move into the new home in Clybourne Park despite the uncertainty and possible danger. Overall, one can argue that Lena's words and Travis's presence motivate Walter Jr. to experience a dramatic change of heart at the end of the play.