Trust

I don't understand myself when it comes to the issue of trust. Some people I trust intimately and immediately...I share with them things that are quite personal and I suppose most people would be a little troubled to talk so openly about them.....not saying that this has hurt me so far.....if anything it has helped me. My confusion is more regarding people who I suppose I should trust but I really don't.....I can't understand why this is so.....