For me, it's always been a matter of it just taking a lot of time.
I finally made a close group of friends my senior year of high school, and those are still my best friends.
At the first university I went to, I made no friends. I was in the dorms and just couldn't figure out how to socialize with people and be friendly. I ended up being the guy that fixed the computers, but that was the extent of my socialization. I left after two years, thinking I could start fresh somewhere else.
At the university I am at now, the people are much friendlier and easier to get along with, but I still don't have any friends around these parts. Here it is both easier and harder, as I am in my own apartment, so I don't have to constantly watch other people socializing, which makes it a lot easier for me, but I don't interact with people much outside of class and work, but those interactions are mostly positive and I may eventually get a friend out of it.
So for me, high school was far easier, for the scheduling reasons already mentioned. In high school, I'd know who I was seeing and I'd see them five days a week. With that kind of repetition, enough familiarity grows that I can eventually make friends (it may take almost the entire school year, however). In college it seems like whenever I'm on the verge of making a friend, it's either a class that only meets once a week so I don't see that person often enough to make friends, or the semester is almost over by the time anything potentially meaningful comes and then it's too late and you never see that person again.