Changing your mind all the time

I've been with my boyfriend for years, and some reason the relationship isn't moving forward, it feels like we've come to a stand still, I would of thought by now he'd of at least wanted to get engaged or something, but everytime it gets brought up, he just goes maybe one day, or if we talk about having children he does one minute then the next he doesn't no what he wants or he's not ready. I don't get it. I don't get how you can chance your mind all the time