I'll throw my hat in the ring as to what might be causing this. I am turning 30 years old this year, and in my experience, I was probably happier prior to graduation from university. I think there is something deeply unsatisfying about the structure of modern adult life - mostly how and where we engage with work.

See, in university we were in close contact to many people, in our age range, with our interests, in both academic and recreational contexts. In work, we are strictly there in professional contexts. That's not to say you can't make friends from work, I do have several people I consider friends that I met like that, but none of them live near, so spending time with them is not going to happen on a regular basis.

The main way I see people involve themselves with others seems to be through what I'd describe as "activity groups", could be the gym you go to, could be a structured class like dancing or tennis clubs, whatever. But these things are usually at most, a few times a week, for about an hour or two at a time. Nothing compared to what being at university with your peers for multiple hours every day was. I think that physical presence near other people is a hugely important driver of establishment of friendships and social groups.

Plus pretty much all of these things require you to invest additional money towards (usually in the form of a monthly bill), just to access. I didn't have to pay anything additional to join a club at university (of which I was involved with probably close to half a dozen, even if I didn't stick with all of them for all 4 years of my time there).

I probably would feel less isolated if I lived closer to my existing friends, but everyone has spread out a lot and there's not much I can do about that. The new friends I've met are usually not that (geographically) close to me either. Everyone is a 30min drive or farther away now it seems.