The inability to let down your barriers and allow someone else into your life is an automated response built up during your childhood. It’s normally the result of unexplained trauma. The subconscious mind has put in place a border that will not be easily passed, and should someone get passed your walls, if they abuse the privilege they’ve been given the walls will re-emerge and push them outside of the danger zone once more. Can the person’s conscious mind change this outcome? That’s a question each individual will have to answer.

The reason it happens is our subconscious minds prime purpose is to keep us safe from perceived threats to our peace of mind. If you’ve been let down by people when you’re young and innocent, it will leave a scar that no amount of reassurance will remove regardless of how much time has passed. If you’ve experienced an idyllic childhood and the trauma happened after puberty, there’s a very good chance you’ll go from one relation to another without losing hope of a happy ever after.
Your subconscious learned the hard way that if it didn’t allow you to put yourself into a situation where you had to rely somebody else, you would be safe. This, however, is a double edged sword because if the barriers are that strong, nothing and I mean nothing gets in or out. You don’t get to experience happiness or sadness, and if you do, you don’t have anybody to share it with because it’s ingrained into your psyche that other people can’t be trusted with your emotions.
They can’t be trusted with your emotions because you’re not prepared to let them see your vulnerability.
Can someone with trust issues ever have a fulfilling relationship with another person? The answer is yes, but that’s a story for a different day.