It's hard to trust once you learn that people, after a point, stop caring. It's not to say that all people are horrible and wrong and awful; it's just a part of human nature to get distracted, to find other things we feel are more worth our time. I stopped trusting people because I was severely depressed, in the closet, suicidal, and friendless, and everyone else went on about their lives, either not noticing, trying not to notice, or not caring, and I knew that even if I did show others how deeply I was hurting, eventually, they would stop caring. They'd tell me I was making a big deal out of things. That all the pain I felt wasn't worth crying over, and since everyone feels pain, it's not something we should worry about.
But even if people are cruel and hurtful and get distracted too easily, I couldn't find it in myself to stick everyone else into a category, to assume that I already saw right through them. No, we're not all perfect. Humans suck. But you can't focus so much on the bad things that you forget the good. Just because other people are scared to trust doesn't mean that you have to do the same. There are people out there who want to be trusted. You just have to look a little harder.