Every so often I set out to "make the rounds" and see my friends who are spread across the country. I'm a bit of a nomad and yet its often my return that triggers close friends to see each other when they live minutes away. Just pick up the phone and call someone from time to time, let them know you give a shit. It's amazing what a phone call once a month and a visit once a year can do to sustain a meaningful relationship with someone.
People are, on average, terrible at staying in touch with one another. Be the outlier, you'll make the world a better place.
If you want to develop new friendships I'd say find hobbies where you might find interesting people. Some hobbies tend to have more welcoming communities than others, climbing gyms being one of the better environments I've come across for meeting new people.
I play badminton, and I've occasionally played ultimate frisbee, and both have a great inclusiveness culture, at least in Colorado, Washington, Texas, and California, the places I've variously played. Much of my current group of friends that I haven't met through geek-oriented meetups I met playing badminton.
I also used to play volleyball, but ... well, too many jerks turned me off. Now I only play with friends at parties.
It's worth emphasizing that making new friends takes time. It was a year before I felt I had good local friends after moving to Colorado, and 2-4 years before I was doing things with them outside of our original meeting group. It probably could be done faster -- I suck at the whole social thing -- but for the poster above who claimed to be stuck at zero: Put in the time, do the work, take a chance by reaching out, and you can build friendships.
ALSO: Watch this TED talk:
New relationships require for you to put yourself in a position where you meet new people. It also requires for you to have opportunities for people to hang out with you.
I don't have a perfect solution, but I've found a few tricks. I've found that clubs and volunteer events tend to be full of friendly people who will introduce themselves. It also helps me socialize with new people when no one else knows each other.
The optimal number of friends when we were hunter gatherers might have been 150, but the optimal number of friends for a developer who gets asked to do work by a new person every week and contacted by a recruiter twice a day and doesn't have much use for any friends beyond graphics designers is probably much less.
If you don't see any inherent value in having people to talk to, experience things with, and in general unwind with than you are either young and naive or a totally different animal than most of your fellow humans. In the latter case, feel free to disregard my message as it doesn't apply to you but if you are are in the former camp, I'd recommend you think long and hard about how you value people in your life.
Why can't we just let things happen naturally? Why does there have to be a formulaic approach to everything? Why do you need to be an outlier? Why can't people just be normal?
What weight you give this depends on your definition of "supposed to", I suppose; personally, it's been greatly rewarding for me to maintain my friendships, some of which are almost as old as I am.