As you've always got that group of good, unconditional friends to fall back on and therefore you're not as bothered about making friends from work/school run/mum groups/hobbies?
I had a lovely group of friends at primary school and for the first year of secondary school. We then moved and I started a new school at the beginning of year 8 when friendships had been very well established and no one wanted me as a friend. I was also bullied throughout the new school years and so just didn't really leave school with many friends. The town I moved to/live in now is the kind of town too where everyone knows everyone else and people stick with who they know.
I have always struggled since then to fit in in any groups of potential friends, and always find myself on the edge of things. No one is ever particularly bothered about being friends with me.
I see aquaintances that have a solid group of friends that they've been friends with since they were 5, and all lived near each other growing up, and they seem to collect additional friends wherever they go as people seem to admire them for being part of a tight knit group and want their friendship. It's hard to explain but I know what I mean!