Trust isn't something that is given freely and when it is broken, it's hard to ever have confidence in someone again. In the first place, there isn't many people in this world that one can trust. I'm sure one can actually count all of them on one hand alone.
Some people have a tendency to blame their friends for something to cover up their own guilt.
Some even talk behind a person's back because they are jealous of that particular person's perfection. They resent another's achievements because they can't do it themselves, and they downplay its importance and excellence. It's malicious, not to mention spiteful. Actions like this may lead someone to be plagued by insecurities. After all, when one wants an honest opinion, they look to a friend for help.