I believe strongly in the friends come into our lives for a "reason, for a season, or for a lifetime" concept. I've had a lot of friends through different reasons or seasons - participating in a shared hobby for example - but where there's no lasting basis once we move on peacefully.
Having said that, growing up, I always had lots of friends. So it was a huge shock to me as a young adult to realise I was clean out of friends. The combination of relocating 3 times in 6 years (big moves e.g. 3000 miles) plus taking friendships for granted had left me alone. I was also severely depressed at the time which didn't make me conducive to forming new friendships. I don't think I've actively broken up on any friendships over the years, more just let them go from lack of maintenance. I also had lifelong a fear of appearing weak or vulnerable which caused me to have a very confident, assertive outer shell meaning people see me as a very confident, friendly, outgoing person not realising I'm actually desperately lonely.
I started my first fulltime office job at 19, and I found the easiest way to make friends was through work, but that had added complications of work and friendships becoming too mixed. Over the years I've joined groups, I've brought books on improving my social skills, I've actively pursued the "friend" thing. And I personally think it's really hard making friends as an adult. And it takes effort - I finally realised nearly everybody is in the same boat, waiting for others to make the first move. I decided to try to become the person who makes the effort, even if it meant getting rejected. Having a baby is what's actually finally lead me to a new social circle. At the hospital "new mothers" get together I formed bonds with some ladies whom I actually have a lot in common with. And now, a year on, we get together for girl's nights (kid free!), we text, we share our thoughts and feelings. We've formed genuine friendships and it's awesome. Honestly, it's been years since I've had friends this close. I believe some of them will definitely become lifetime friends. The baby thing is a red herring - what really connected us was actually sitting down in a group of people to introduce ourselves and talk, and from there identify who we got along with. It's so rare to have that kind of opportunity in our busy lives to meet new people! (Everybody always says hobbies, except often the focus is on the hobby and there isn't always time to get to know the person).
I've also made an effort to reconnect with my childhood BFF (we had a falling out as teens, mainly exacerbated by my relocations and a few misunderstandings between us over the distance). That is a friendship I will have my entire life.
I plan to teach my children to value their friends and not take relationships for granted. Ironically, I've never taken romantic relationships for granted - even as a child I studied relationships to try to see what worked and what didn't, desperate not to end up in the same miserable state as my parents. And that knowledge has lead me to form a very healthy, happy marriage. But it never occurred to me that such efforts should also be applied to platonic relationships!