Hmmm... I don't know... it just hit me at some point.
I first started feeling somewhat ready when I was 26 and moved from my first job to my current one. I always felt that my first job would be temporary and while I was open to marrying during that time, I worried that I would get stuck in a certain place and in a certain job if I married then. Plus, as a dating late bloomer, I liked having the chance to date a lot with no expectations.
So, during that period (ages 22-26), I didn't want to marry, didn't feel ready and that probably affected my dating choices or sent out a vibe to various men that I was a footloose and fancy-free single.
When I moved to Job #2, that was my first sign to myself that I was kinda ready to settle down. But I REALLY felt ready when I noticed how frustrated I got with men who wanted to just date (usually older, divorced men) and not commit -- before I didn't really care. Plus, peer pressure can definitely play a role... when you see your peers that you used to run with suddenly marrying and getting all into mommyhood, you start to feel that urge yourself.
Sorry, but the idea of chasing dudes and dressing up to go to clubs all the time in my 30s was NOT cute! And I noticed the priority shift and even a sense of maturity in my married friends who were now parents, and I thought that would be good for me. Eventually, you just get tired of being single.
I didn't have a mate at the time, but when I met my now-fiance, I made it clear early that I was dating for the purpose of marriage. That's all I needed to do to show him that I was ready!