There's a fine line between being personable/outgoing and being abrasive. I mean that in the nicest way possible, but you all know what I mean. There's always that one person who walks around campus, acting like they are entitled to everything they want.
For clarification, upperclassmen do not dislike underclassmen. The days of seniors stealing your lunch money and stuffing you in lockers were over when you shook your principal's hand, took your diploma, and fell down the stairs at your high school graduation. In fact, most of my best friends are younger than I am, and I wouldn't change that for the world.
What is true is this: upperclassmen are tried and true. They've had the opportunity to make great strides in their academic area(s) of interest. They've had the opportunity to participate in countless extracurricular/cocurricular activities. They've had the opportunity to participate in internships and externships related to their future goals. They've had the opportunity to build up their social prowess. The point is: They've done it all.
To current freshmen... You are just now limping to the finish line of your first collegiate year. Congratulations. But, in the grand scheme of things and to put it frankly, you have a ways to go. Yes, you know what being a college student is like at the most basic level, but it only gets more difficult from here. Completing a single year of college doesn't immediately grant you a Ph.D., an MD, etc. You're most likely not the most intelligent person on campus. You most likely don't know everyone. You most likely haven't reached your full potential. So, stop acting like it. By all means be proud of your accomplishments and pat yourself on the back because you have worked hard for what you have earned, but realize that your college experience doesn't end after your first two semesters and that there are those older than you who have experienced far more and have achieved far more. Use them as models to remain grounded and focused. Find someone older who can take you under their wing and show you the ropes; you won't regret it.
To incoming freshmen... Don't be "that guy". You know, the person who walks around campus bragging about all of his/her high school accomplishments. That doesn't make for a great first impression, and truth be told, your high school accomplishments don't automatically give you an A on your first college exam. Imagine yourself running the same race twice--let's say it's a mile. The first time you run the mile-long race, you're wearing a backpack full of feathers. The second time, the feathers in your backpack are replaced by a fully-grown adult, male elephant. You're obviously going to have a much easier time the first time around, and the same is true about the relationship between high school and college. Yes, it's also true that some of us have a significantly more challenging high school career than others, but even the most challenging high school curriculum doesn't properly compare to what the college academic world is like--not to mention that college also marks your first true step towards adulthood. There's more to college than getting up and going to class. You actually have to study to do well. You also can't be involved in twenty clubs like you were in high school. Use the five-finger method, each finger representing a different aspect of your life (for example: academics, family/friends, Greek life, work-study, and a club). Don't bite off more than you can chew, and don't be afraid to admit that you can't handle it all.
To both current and incoming freshmen... Try to slow everything down. Yes, I've been advocating for growing up and becoming the upperclassmen I know you can be, but slow it down; you'll get there one day. As a rising senior, I wish I could go back and have the chance to re-experience my past three years, but instead, I will live vicariously through all of you. Live it up, humbly. Enjoy it. It goes by too quickly.