Why is it hard for a person to trust people?

Perhaps if that person has been let down by people in the past or had an unhappy childhood, or parents that did not care so much for them - many factors can be the cause of some one not being able to trust others, but not every person is the same one must learn to try and tell the people that can be trusted as one can't go through life shutting everybody out the person that does that will end up very alone.