I think part of the problem is that during our childhood and school years we’re forced into making friends with other children, be they other kids in the neighbourhood, kids we had to sit next to in class or other relatives our own age. How many times were we told to place nice with others or to learn to share? We almost form a dependency on our peers at a young age, in particular when belonging to part of a social group to help establish our own identities.
Once adulthood hits all that gets thrown out the window. We don’t have to speak to our neighbours, we don’t always have to interact with our colleagues unless the job requires it and we’re certainly not forced to hang out with our relatives when a brief “I’m too busy to attend” excuse will suffice. A lot of adults, myself included, learn to become completely independent to the point where they don’t really need others to carry out a vast majority of functions within their lives. You’re no longer forced into relationships and as bad as it sounds we’re too lazy to try otherwise. Why bother to invest time in making new friends when you have the old ones still around?
The problem then is that when you get to adulthood without those childhood friends to fall back on you’re either stuck within a likeminded “I don’t need to make friends anymore” demography or those who have gone the opposite end to create their own family unit which they rarely have the time or desire to look beyond. At my age I’m in no hurry to date or romance but would love likeminded friends to hang out with, watch a movie with or just chat to about this and that. Alas as I said, most people my age already have older friends to do that with and can’t be bothered (or have even forgotten how) to start playing nice with others again.