Well, I don’t know about anybody else, but I WANT to get married eventually. In fact, I want to marry my SO, the most wonderful guy on Earth. We’ve talked about it, even about ring shopping, though we figure for several reasons it’ll be a while before we legally do anything. But if he decided after getting engaged and all that he didn’t want to marry me, that would be that.
I think for most people “I don’t want to marry you” does equal a breakup, once you’ve gotten serious enough. Why waste my time with somebody who doesn’t want to settle down with me when I want somebody who does? Sure, it’d half kill me to lose him, but in the long run, I’d survive. I did once.
For a lot of people, dating is part of the process of finding a life mate. It may be a legal marriage, it may not be - but it isn’t a short term “hey, I’m happy you are here now” sort of arrangement, its a “I want to know that - barring tragedy - you’ll be here for me in the future.”
Age and maturity have something to do with it. Life mate shopping at sixteen may not be the best course of action. And maybe when you are 76 you stop worrying so much about whether the person you are with is still going to want to be with you at 80.
The desire for kids has something to do with it. If you want kids (which implies either marriage or a long term stable "I’m going to like you for the next 20 years) and don’t want to single parent, then you are wasting time with someone who doesn’t want kids or doesn’t want to be with you for the long haul.
Some people don’t care for long term interdependence, but I think most people do want to find someone who they think they can establish long term interdependance with - at least at some stage in their lives.