Eh, what else would you make friends from?
Well, anyway...
Since the word "friend" encompasses quite a few concepts, I'll clarify that what I'm about to say applies to "active hang-out" friends (you hang out with these people regularly, for leisure, even though it requires going out of your way) and "support network" people (to the extent of your ability, you will help these people when they are having trouble or higher-than-normal workload in their lives). The same people can be both of those types. There are other types of friends to which this does not apply, but I feel like the OP is probably talking about "active hang-out" friends.
I think most people have a cap on how many friends they want. I mean, no one ever thinks things like, "I have 5 friends, so I refuse to make any more," but friends take time and personal energy, and sometimes hanging out with them involves spending money. All these resources are limited, so you can't just keep adding friends willy-nilly. So once someone has reached their friend limit, they will naturally become resistant to making additional friends, possibly without even realizing it or consciously thinking about it. If they try to go over their personal friend limit, they will have to sacrifice something else in their life, which possibly means downgrading one or more of their current friends to "pal you don't actually spend much time with" status.
With that in mind, if you're trying to become friends with someone who is already at their personal friend limit, it's going to be really hard. You have to be so freakin' awesome that hanging out with you seems to be worth the sacrifice it will require. That's why it's so hard to make friends when you're the only new person in an established group.
If you're doing everything right, but people just don't have room for you in their friend pool, you will probably end up being their friendly acquaintance (you are nice to each other, but you don't spend much time together beyond what your school/work/whatever requires). If people are actively mean or hostile toward you, that probably means something about you or your behavior is making them dislike you--which is not necessarily your fault.
If you try actively seeking out people who have less than their maximum number of friends, you might have better luck? Unfortunately, there's not an easy way to learn this information about someone. Maybe try looking for people who aren't married, don't have kids, aren't always complaining they're busy, aren't taking lots of classes at a time, and don't have a job in addition to their classes? Bonus points if they recently went through a major life transition such as being a new student at the college.