People get married for different reasons,at different ages.

 I have been married, had 2 kids,thought I had a great marriage, when she jumped the fence,and went to greener (weed patch) pastures. While raising my kids, I dated several women, with no intention of getting married. My kids were too important to me, and being a single Dad,working 2 jobs and farming part time left little time for a serious relationship. After the kids left the house to college, I started dating with an eye towards my future. Now farming full time,and have lady who lives with me, neither of us talking about marriage, after all we have 36 years of marriage between us (just not to each other)

 Now, her kids are all above 18,mine are doing pretty well in the world, maybe one of these days the subject will seriously come up, however for me it is not about making her mine,while alive, more about taking a little care of her if I was to be "gone". Like I said, we marry for different reasons at different ages. When I was young, I wanted a woman to build a life with and have a family, now it is more about companionship, life has been built. Maybe it is more about happiness, and having someone to travel with,dine out,going out with other couples and such.

Like someone said, there may be something in her past, which keeps her from wanting to commit. In my case, my previous marriage was good until it ended abruptly, hers, however was bad from day one and just got worse as the years passed, so obviously,I have more reason to look fondly upon marriage than she does. Regardless of that, a relationship will take it's own course, and forcing any issue one sided will not help.

Good luck