While being an underclassmen is most certainly not the place you want to be all of your life, most upperclassmen will admit that they miss it. Rushing through your first two years of college to finally get to your major classes and be able to legally drink aren't all they're cracked up to be.
Gen Eds
Remember when you got to skip class multiple times a week without your grade dropping? Remember lecture halls of 200+ people with no attendance? Take advantage of that while you can. Soon enough, you'll fall behind just from being 5 minutes late, and your professors will all know you skipped class before you even did it.
Getting to be on campus
When you move up past Gen Eds, you'll become a prisoner to the building that houses your major. If you're like me and are in a studio major, that means you never get to see the library or lecture halls or your favorite tree on the quad ever again. You'll make excuses to go anywhere but there.
Dining halls
Ok, so they're not that great -- but think about how much you hate cooking, and imagine having to do it every day. Thank your lucky stars that you get to show up any time and have a plethora of food completely at your fingertips. Think about the fact that they never run out of ice cream. Think about chicken nugget night. Yeah, I know, I miss it, too.
House parties
When you're not old enough for the bars, house parties are the only real option. When you have no idea what you're doing, but you stumble into a house surrounded by a ton of sweaty people you don't know, and there's jungle juice, and you don't even care that you're too young to get into the even-sweatier bar. Those are the foundation to the best nights of your life in college (and, admittedly, sometimes also the worst).
Meeting new people
When you've gotten to the point in school where you have all of your classes with the same 10 people, you rushed two years ago and you don't live in the dorms anymore, you've basically made all of the friends you're going to make. I'm sure they're all great, but meet as many people as you can while you still get the chance to.
Living on campus
As much as it totally sucks to live in dorms, there's nothing like having all of your friends only steps away and not having to find parking on campus. You can see everyone you love every single day without having to worry about what side of town they live on, and it's way easier to help each other with projects and homework.
Cherish it while you can, noobs.