This is describing me. I don't need people so never really sought them out. It's only as I get older that I realised how isolated I was and now I freelance from home I don't even have work friends to socialise with.
It's taken real effort to be the one chasing friendships rather then sitting and waiting for someone to come to me. I'm slightly anxious so that always made me hesitate to make first move as I was always uncertain if they liked me or not.
Even today I've been thinking on contacting an old work friend but putting it off in case he's bored of me or something.
There's been a couple of people that I have liked so much that I have put a lot of effort into and those friendships have maintained, all be it sporadically.
My advice for maintaining/starting friendship.
Suggest 'getting coffee sometimes' or ' let's get the kids together' or 'oh I'd love to see your orthodox russian iconography" (or whatever they are working on). Don't let them be the one always chasing you.
If they even half suggest you should do something. Take them up on it.
Ask them questions and express interest in their lives, ask follow up questions and show the appropriate emotions. Don't just talk about yourself or immediately say 'OH that happened to me let me tell you about it!. If you do the above keep it short!
Smile and wave if you see them in the street. (I blank people terribly)
If they ask how you are. ASK THE SAME BACK.
Don't get personal too quick. If you are on a long train journey together, yes you can share your long family history but not at the school gates. Leave that for small talk or moaning about work or the traffic.
Find weird niche hobbies and join a group. The weirder and nichier the better. I'm part of a fantasy novel writing group. It's the only group where I have met real friends because they have brains like me. We nerd out on magic systems and writing technique.