Final answer:
Walter is upset about Beneatha's medical school plans due to the financial burden and conflict with his own investment aspirations for the family's economic improvement.
Step-by-step explanation:
Walter is upset with Beneatha's plan to go to medical school because he sees her ambition as a financial burden, particularly given the family's strained economic situation. From Walter's perspective, the cost of medical school is prohibitively high and the likelihood of success does not justify the risk given the other immediate financial needs within the family. Beneatha's dreams of becoming a doctor conflict with Walter's more immediate desires to invest in ventures that he believes could lift the family out of economic hardship more directly.