Late night phone call

For the past couple of weeks I've been hanging out with M and his 2 friends. One is the girl that I met at the bar 2 months ago and the other is her friend who M is talking to. We all went out last night to a bar and were hanging out. I was a little out of it because I had a long day. The one M is talking to is very pretty. After I got home yesterday she gave me a call to thank me for coming out with them last night. We ended up talking for quite a while. She was saying how I have been so quiet around them.
I have been quiet around them and the conversation led to an explanation of why I have been like that. That got me thinking. I guess basically what it comes down to is that I have a lot I am trying to focus on right now. Particularly myself and getting my education and career on the right track. It is hard for me to enjoy myself, have a good time, and open up to people when there is stuff on my mind. I am more closed off and don't let people in easily. I just have my own things to focus on right now. Is that so bad? Am I being selfish in not getting to know other people and not even spending that much time with my own friends.? Perhaps I am however I think it is something that I am working on overcoming.