I know you, but I don't know you

I was talking to my friend, J just now. Halfway through the conversation, he said, "Sometimes, I feel like I know you but I don't know you. With most people, I figure them out pretty quickly. It's like your mind is running around all the time. You're always thinking something but I can't figure it out."

I paused for a couple of seconds and then I replied, "Hm, yeah. I get that a lot. I'm not the most open person out there unless I know you pretty well."

This brought me back to the time I had a conversation with another friend a couple of years back. I think he felt a little frustrated because he said I'm not open to him and I placed an invisible barrier between us. I would tell him my dreams, goals, worries and fears but still, I didn't allow him into my world.

It usually takes me years for me to be really open up to another person but when I do, I'm extremely open and fiercely protective of my friendships. Sometimes, fortunately, it doesn't have to take that long. It may just take one meeting and I'll feel an instant connection. However, it's difficult for me to feel that way. So I sit here and wonder, why am I so reserved when I first meet a person? Or is that a good thing or a bad thing?