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

I believe that Beneatha does so for a couple of reasons. 
First Asagai makes Beneatha realize that she is self-centered.  While Asagai dreams of
returning to his homeland and making it a better place, Beneatha flits from one hobby
and college major to another, trying to find something that pleases her and disregarding
how she could use her intelligence and education for the benefit of
others.

Secondly, Beneatha's life experience had caused her
to be cynical.  She has watched her brother Walter struggle with his own dream.  She has
no respect for him and think that his goals are worthless and unattainable.  When
another male--Asagai--discusses his dream with Beneatha, it is natural for her to doubt
that he will accomplish his goals because she has not observed anyone else doing
so.