Well, first, they’re generally more intense.
Think about it. In high school you see your friends or boy/girlfriend for 6-7 hours during the day and then during whatever time you specifically plan to. You don’t live together, and you only occasionally eat together (aside from lunch).
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.
The plus side is that if you can endure these things you may make friends for life, there’s always someone to talk to, and this much closeness offers the possibility of great bonding with people you may have not met otherwise!