In college you have the potential to see a person almost 24 hours a day. You might live in the same room or across the hall from them. You might share a bathroom AND classes AND clubs .with a person. Meals are taken communally so you might eat together too. Because you have the potential to spend so much time together, often romantic relationships move faster than they do in high school.
Friendships get more intense, too.
It's also harder to get way from someone who is annoying you, because they always know where to find you. Or, if you are in an argument with someone, it can be hard to avoid them. Actually, it's easier. I felt some peer pressure to drink in high school. Some people are jerks and may pressure you to do something you aren't comfortable with.