(Let's disregard the probability that you were never really independent - it was just an illusion that you allow to flatter your sense of self).
What happens is: you start having obligations. 'I've got to go to this thing because X has Y connection with A, B & C.' At first, they're small blips in an otherwise self-directed life. Then one day you turn around and notice: going to parties/things with people you barely know is your only social life.
Moreover, you realise you have known these people for a very long time and see them more regularly than people you're ostensibly closer to. Then you think: 'But I'm not actually close to any of them. These are just familiar faces in a room. I'm not even sure I like this room. I prefer the one at home. Just because I recognise these people doesn't mean we have anything to talk about. In fact, I think we exhausted our conversational reserves sixteen years ago, when we met. Oh God, I'm hungry. Better drink more. No, that's not a good idea. Too late.'
So what I'm saying is: does anyone else ever find themselves thinking: 'this is like going to a Christmas party for a job I left five years ago'?