I don't think people realise how much effort it takes to make friends and keep them. I returned from living abroad as a teenager with no friends. My close family remained living abroad so I didn't even have them. My school friends were in a different country. I had to build my friends from scratch.
Over twenty years later I now have loads of friends. Several large groups and a lot of individual people I've collected from workplaces, through kids, through old boyfriends, college courses.
I've always made an effort to meet up, ask them how they are, remember special birthdays etc. I put time into being a good friend.
I would say I still don't have a best friend but I'm not really that sort of person. I prefer different company for different occasions, walking, dancing, going for cocktails etc.
You just have to realise that sometimes you have to make the first move. It's usually me who organises 80% of the social meetings but I don't mind as people jump at the chance once you ask them normally. If people say no then I don't take it personally, I just move on.