When do you let your guard down? When we meet people, it is natural to not give all of yourself to them. After all, you don't know if the person will be a hit or a miss. After some time you can tell that someone might be worth letting your guard down for. The worst feeling in the world is showing your vulnerable side to someone who couldn't appreciate it.
The reason most people don't let their guards down is because of fear. Fear of pain, dishonesty, deceit, infidelity, rejection, disloyalty... the list goes on. These feelings stay with you until the person you're dating melts walls and makes it easier...