I've always identified with Jo, from the first time I read an abridged version of Little Women when I was seven or eight years old. She desperately wanted to be a boy instead of a girl, and so did I. Her best friend was a boy, and so was mine (until I was 8 1/2, anyway). She was a writer, and I already liked telling stories, though I didn't identify myself as a Writer until I was about 14. She liked writing and acting out plays, and so did I. She whistled, got told she wasn't ladylike enough, and felt generally irked by societal constraints, and so did I, though I wouldn't have put it in quite those words at the time.
I think that anyone who has felt awkward, out of place, or annoyed with the way the world works can sympathize with Jo. She wants to be accepted and liked, but she doesn't want to have to do things the way everyone else does just because that's how they do them. She spends her adolescence feeling at odds with the world around her, but in her young adulthood, she finally learns to accept that she's different and find people other than her family who appreciate her for who she is.
When I was a young girl, I struggled with the difference between how I wanted to do things and how I was told I was supposed to do things. An officious stranger once told me I couldn't sit backwards on a chair with my elbows resting on the back because that wasn't ladylike -- it was cowboylike, and I wanted to be a cowboy, not a lady. I liked playing with cap guns more than Barbie dolls. I flipped pancakes "backwards" and tied twisty-ties "backwards" and didn't smile for pictures when riding merry-go-rounds because I was too busy imagining I was a cowboy.
Like Jo, I was blessed with parents who generally accepted my personal quirks and let me live my life on my own terms for the most part. Marmee March, in particular, seems to sense that if she doesn't push Jo to conform for the sake of conformity, Jo would eventually stop being contrary for the sake of contrariness. At the beginning of the book, Jo behaves in unladylike ways specifically to annoy her sisters, but by the end, she has dropped her unconventional habits like saying "Christopher Columbus." Yet she still does things like accept a mixed-race student into her school despite people insisting it's a bad idea. She marries Professor Bhaer, a poor man wildly older than herself and not generally considered a "good match."
Over the course of the book, Jo learns that living life your own way doesn't mean doing things the opposite of everyone else all the time, but only when it's necessary. That's a lesson stubborn me is still working on, I'm afraid.