I also went to an all-girl's high school. I loved that we were there for each other and for ourselves. None of that petty HS crap that I went to a signle sex institution to avoid! When I got to college, I wanted that experience to continue. I knew I wanted to be Greek back in HS, it was a matter of finding my fit.
Sorority life is about sisterhood, a band of women who live for each other, care about one another, and grow and learn together for the greater good of the organization. Personally, I think that is the definition of feminism.