This is such an interesting thread to read. I love seeing other people's experiences with this.
I personally think it's down to personal choice, lack of opportunity to make friends, not having a personality/energy that people naturally gravitate towards, or not being good at maintaining friendships.
I'm in my late 30s, very warm/friendly - but I'm an introvert. Any new friends I make tend to be extroverts, which means I often end up feeling suffocated by constant texts or invitations to do things.
I have two best friends, who I've known for 15-20 years. I adore them. I'm also very close to my sister. Our connections work because we're all similar - we like our own space, we don't feel the need to constantly talk, and we're really flexible.
It's impossible to replicate the depth of these connections, because there are already so many miles on the clock. That's kind of what I struggle with, with new friends. New friendships feel exciting to me at first; but after about 6 months, I usually get bored. There is not enough depth for me, or something. I just start to feel completely uninterested in texting or meeting up.
I tend to find my activity/proximity-based friendships quite shallow. At first, I enjoyed having new people to do new stuff with. But after a while, I start to realise that we're too different. Like, they might be more materialistic, or not as open-minded, or something else.
I'm moving to the coast in a year or so, and honestly, one of the parts I'm most looking forward to is getting away from the social buzz of the city. I just want my time to be my own, and I really love the idea of being able to go a few months at a time without going back to the city to see the people I do want to see. I often wonder if I'll feel differently when I move, and whether I'll feel lonely without lots of people I know in the area.
Basically I'm a lazy, ungrateful friend :p