It’s not just about who you are. Modern life is full of instability, but most relevant here I think is future plans. It might make sense to be together today but career paths, opportunities, desire to live near family or friends, things that we want, can all change with time. This is especially true for young people today, with the unprecedented income inequality and expense of basic housing. When you’re just trying to get your feet under you it’s hard to imagine getting married, and you don’t know if you’ll still want the same things in a few years. It’s not apathy, it’s pragmatism.