It tends to be easier to have friends when you're all living close together and socialization is very easy. I run into my neighbors outside all the time and we regularly chat with one another. Then every month or two someone throws a party and invites everyone else over. That's how we all stay connected.

I've got another group of friends who host a game night at their house every other Sunday. Its a routine, same time and same place, and the house is right in the center of town so its easy for everyone to get there.

I almost never see my coworkers outside of the office, though. They're all out in random distant corners of the various suburbs. Any kind of meeting would be a huge hassle for at least some of them. We grab lunch on occasion, but we don't have any opportunity to see one another outside of work, so friendships never develop past basic business associations.