In A Raisin in the Sun, why does Beneatha attack Assagai's dream about Africa?

Whenand
Joseph Asagai converse, as her family is packing up to move to their new house, she tells him
that she will have to give up medical school becausehas given away their money. For the first
time, she tries to explain to him why she wants to be a doctor. Beneatha admits that she is an
idealist and is worried because just wanting to do something cannot make it happen; desire is
not enough. She accuses Asagai of thinking childishly, as she used to think.

BENEATHA. . . . You with all your talk and dreams about Africa! You
still think you can patch up the world. Cure the Great Sore of Colonialism(Loftily, mocking it)
with the Penicillin of Independence!

ASAGAI. Yes!

BENEATHA. Independence and then what? What about all the crooks and thieves and just
plain idiots who will come into power and steal and plunder the same as beforeonly now they will
be black and do it in the name of the new IndependenceWHAT ABOUT THEM?!

Beneatha is taking her fury at Walter and the loss of her future
that went with the money and directing it toward Asagai because he is so much like her. She
would prefer to reject all idealists rather than have an example in front of her. Her anger dies
back when Asagai suggests that she go to Africa with him.