How come I'm able to talk to some people fine and carry on a conversation but with others I just stay quiet?

I'm exactly like that!! Wow, and I'm in high school too and I barely see my friends either and I feel like I have no friends. There are some people who I feel comfortable talking to and others I feel shy around. I've noticed that I feel comfortable talking to people who are quieter/shyer than I am and I feel less comfortable talking to people who talk a lot more than I do.

this happen when you feel comfortable with others.

It's only natural that you don't talk well with people you don't know very well. You're better than me; I sometimes have what I think of as an anxiety attack when I'm nervous about talking to someone. You just have to realize that not everyone likes you and go on with it. You can't change the way people feel about you and, trust me, after you assert to people that you are confident you will find that conversations are much easier. To tell you the truth, I don't even see more than half the people I used to hang out with in middle school. Time changes all of us and we acquire different sets of values as we grow up. Our shifting intrests also divide us. For example, while you might have liked skating in Junior High but an accident that was life scarring could cause you to hate it in High School. It is completely natural to lose contact with people you used to be in tight with. It's sad but true.... :( So in a nutshell, it's not your mood it's the ever-changing you.

You feel more comfortable in familiar patterns and settings. You need to start listening to others and finding what you can learn from them. Conversation proceeds pretty naturally from curiosity.

Some people are just quieter and shy around others.... my FAMILY think that i'm nuts, but at work and in uni i would hardly ask for directions cos i don't wanna talk to people You are just the way you are :)