Gf (at the time) asked me to marry her legally while hiding it from her family

I declined at the the time our if respect to her parents. She broke up with me a few month later. We were together for 4 years. This happened in November last year and I am still trying to process it. Why the literal obsession with marriage? I was against it one bit, we both had things to accomplish and I felt that was the most important thing at the time. It's like she wanted it her way with zero regards to my feelings/opinions.