Trust is something most if not everyone considered to be sacred. It's one of the important components of the glue that keeps relationships together and intact. When a person loses their trust in someone, the relationship begins to fall apart because the glue that kept together isn't strong enough any more as a fundamental ingredient had been taken away. I remember when I lost my trust in my best friend. It didn't stop me from being her friend but it taught a lot about who to confide in and what sort of things I should and shouldn't confide. It's one of those things where you know how the person is going to react but at the same time, you wanted someone to confide in anyway and see as she was my best friend, I thought that I could tell her anything but instead I was wrong.
I'd rather not go into detail about it as it is a very long and complicated story so in the spirit of keeping it short; I was in an argument with her and my brother. They both believed that I was a weak pillar and a danger to the business because of how nice I am and how 'easily manipulated' they believe me to be. At that point, my 'best friend' decided to use something I confided within her as ammo, more precisely as an example to how exactly she believed me to be inadequate and lacking. A person we work with had taken a liking to me and I shared his feelings but the complicated circumstances of the culture really prevented things from going any further so I had to end it.
It was that moment that I felt my trust in her shatter into a million pieces. I didn't really feel the same after that and my behavior with her was never the same. I grew a bit distant and cold and the next day she was hoping that things would get back to normal so she asked me "I want you to be my best friend again, why won't you talk to me?" to which I replied "The last time I talked to you, you used my words as ammo against me".
Trust is a very fragile and sacred thing in a relationship. Once you lose it, it becomes very hard to get it back.