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.