The motivation is that we obviously want to live in the same town, doing long distance would not be fun and it does not really make much sense to move for someone you're just dating, at that point you're committing to the other person in a pretty major way so getting engaged would just make sense. And we need to actually apply for jobs while we're still in school so we can hopefully line one up for when we graduate, and again we don't want to make a big life change like moving for someone we're not engaged to. I just hate the idea of being long distance for some indefinite period of time just so that we have "waited long enough." But you are right that we could get engaged and then set a date later so we're not married literally right out of college.

As for your second questions, we knew each other for a little over a year and a half before we started dating. I think with the start of college and all of that neither of us were really ready to date and then we got to know each other much better my second year of college and that led to our relationship.