While on the surface a person thinks that in college they are "all grown up", this is rarely the case. Making the transition from being a college student to being a productive member of society, with a real job and all that it entails, is a major change. Many people realize too late that it is hard to make friends and hard to maintain the social life that you had in college because of the lack of opportunities presented to you-whether they be in the form of friends, things to do, or time. Money is the one factor that almost always works out better for you when you've graduated, but even that is not a given.
So since you are making this huge change, it stands to reason that other, more romatic areas of your life will change as well. In college, time is something that most people have in abundance. Classes really only take up a few hours a day (and maybe less in the final semester!) and many people choose to work holidays and summers back at home as opposed to having a job while they are in college. Therefore, with the absence of must-do activities and a real job, you have a lot more TIME to spend with your significant other.
Parents may also be a factor in this change, depending on how close you live to them (or if you live with them!) and how strict they are. Obviously extremely conservative parents would not approve of you spending the night with a significant other, but this is something that you can do to your heart's content in college, away from people who disapprove. This is, of course, with the consent of your roommates and other people involved--dorms are not as fun to do this in! Regardless, you'll almost always have more time and less flack for doing what you want, when you want, with the person that you want to do it with, when you are in college and away from parental supervision.
These are two major catalysts in molding a relationship after college. Surely there are more, but these are two of the biggest ones, and they are certainly something to consider. Do not graduate with the notion that everything is going to be the same (you might not even be in the same town, remember!) and make sure that you both make the proper adjustments to your minds and to your relationship. Real life awaits, with job, housing, food, and money issues; and most of all, time. You WILL get frustrated with one another and with the constraints that life has placed on you, but just keep in mind that the fact that you love one another is the most important thing. Enjoy dating in college while you can, and prepare to make the smoothest transition possible when you leave.