I think it's just about different personality types as well. Personally I'm an introvert which means I love talking to people and hanging out with them but after a long day I get drained, so I need that time after work to myself. And then on weekends I want to spend it with my personal friends.

I'd try asking a co-worker out for lunch or something, if you want to become closer. I found I just needed more time with people before I was able to open up to them, and doing something physically outside of the office environment helped with that. That said, sometimes you connect with people and sometimes you don't. Think back to elementary school or high school--a lot of those people were only 'friends' because you had to be around them all day every day. It's not necessary that they were anti-social or bad, it's just you didn't naturally connect to them like you did your actual friends. Different strokes for different folks!