You don't really know someone, nor know yourself with them, until you live with them for awhile. People get married way too quickly. How can you make the decision to spend the rest of your life with someone, when you don't even know them. Additionally, they don't seem to realize that the infatuation stage doesn't last forever, and if you get married thinking that the rest of your life is going to be like that, you're in for a huge disappointment.
My wife and I were together for 12 years before getting married - and lived together for 8. In that time, I don't know how many couple we knew who met, got married, and got divorced, all in a matter of a couple of years.
People would ask me why I didn't "marry her" if I wanted to spend my life with her. That's backwards. We weren't in a hurry to get married, and didn't need to, because we knew we wanted to spend our lives together.
Basically, deciding to spend your entire life with someone who you really don't know is pure folly.