Is it possible that in general we have more attachment with our colleagues in our earlier jobs than the later ones?

I remember when I got my first job I was extremely social and would try to make friends with anyone. But now that I'm on my nth job I don't feel the need to socialize as much and would prefer to put in my work hours and move on with my life.

This is somewhat true. Definitely at the start of your career you are more gregarious.

I don't think it's a good thing though. Middle aged men especially, I think, generally could do with a wider circle of acquaintances.

I try to keep in touch with a lot of my ex coworkers every 6 months.

Some of them are actually very keen to catch up, years after the job.

I'm friends with people who I was not very friendly with on the job 18 months after I've left the place, after sending them a text message saying "how's it going."

This resonates a lot with me.