For me it was ConLaw as part of the first year of law school. It forced me to relearn US civics, and confront all the gray area that exists in trying to build and maintain a functional democracy. It expanded my view because it retaught periods of US history from a very different perspective than I'd encountered previously. Considering the interpretation of laws and policies through the lens of precedent and separation of powers made things far more complicated and interesting to me. I made peace with the fact that answers that start with "it's complicated,..." and "it depends,..." are generally better (if less satisfying) than ones that start with "clearly,..." or "obviously,...".