The Philosophy of Trust

I have talked about love, I have talked about loyalty, I think it is fit that I talk about trust. I am going to state my question now, and then again at the end. Why is trust so easily lost and so hard to gain? I, like many people, have been hurt before. Boyfriends have dumped me, parents disappointed me, friends have back-stabbed me etc. As the years  have gone on I have found it harder and harder to trust people because of the amount of hurt that I have been exposed to. I know it is wrong to make others pay for the actions of someone else, however it seems like whenever I do finally open up to someone, Boom! I am shot down again. Is it my fault? Do I have crappy choices in friends and guys? I don't know maybe, but all I know is that recently the topic of trust has been on my mind. As stated before I have been hurt before and it has affected my friendships that I currently have. I have slowly gotten over the pain of being hurt and have opened up more to my friends and have slowly beginning trusting them. Through my relationship with others, I have realized that thee are different levels to trust. For instance, I trust my friends not to talk behind my back, but it will be a few more years before I totally and absolutely trust them with my life. I trust my parents with my life and that is about it, but I still have one more person I trust beyond my parents and that is God, because my parents have let me down  before. I trust my friends with secrets more than I trust an acquaintance. So you get the point of these levels of trust, the thing is I just find it interesting.  The next thing that I find interesting is the amount of time that pasts that enables you to trust someone. I am going to trust someone I have known for years more than a person I have known for days (although this mentality sometimes backfires).  Trust really isn't black and white. It really is a complex topic. So my question again is, Why is it so hard to trust someone and so easy to lose trust?