In high school, you're surrounded by the same people every day for four years. You develop friendships, mostly out of chance, but sometimes because you truly like the other person. You hang out with them all the time, go to lame high school events, and your family knows everything about them. Eventually, they'll stab you in the back, or you'll just go your separate ways, only a few actually keep in touch.
In college, making friends is a little harder than usual, but once you do make friends, you will be with them for a lifetime. They share many interests with you, come from different backgrounds, choose different majors, but the differences and similarities are what creates such a close bond. College friends do stupid things with you on the weekends, but help you study for midterms and finals during the week.
Friends are what gets you through the hardest of times, and lifts you up in the best of times. They're essential, and everyone needs them.