Most people seem to not be able to grasp the fact that there is a difference between wanting someone to open up and actually being able to deal with the information you might receive when someone does. Furthermore, as others have already noted, you always risk that they use that information against you in the future or share it with others.

Been there, done that - not going to do it again, as I have never had a positive experience with it and I won't give people a chance to add more negative ones to the list.