Why are intercaste (both M/F dwija) marriages frowned upon? [Serious]

While it is undestandable how/why upper and lower caste marriages are considered blasphemous, why are intercaste (both dwija) marriages frowned upon? Please stick to the point. What is the importance of caste endogamy in Indian culture ? I'm curious, since I've seen many people fixate absolutely on finding a partner from the exact same caste/subgroup, regardless of any other concerns.