Because even though they know how you act they are oblivious to your true feeling and some don't understand what it feels like to be betrayed or betray so you should talk to that person and express your feeling.
It can be many things but it comes down to jealousy the root of all evils. People close to you like friends and even family know everything about you. They know your weak points and your strong points. It's easy for them to hurt you because they know if they do it could be your downfall. Worst pain in the world is having a family member stab you in the back.
If you weren't close to them and they betrayed you, it wouldn't mean much or effect you much because odds are they wouldn't really have much to use against you. Those you've shared personal things with and felt close too, are the ones with the most power to hurt you.
Though we may want to strike out at the betrayer, we need to take our cause to the Lord. "Do not repay evil with evil or insult with insult, but with blessing, because to this you were called so that you may inherit a blessing" (1 Peter 3:9).
There is perhaps no greater insult to relationship than betrayal. Betrayal robs us of a sense of security. Someone close to us has proven untrustworthy. Most of us have felt the sting of betrayal; likely most of us have even inflicted it. Betrayal is a robbing of security through a breaking of trust. We overcome the heartache it causes by giving our pain to God. We call the betrayal for what it is, reconsider our personal boundaries, and recognize that only God is truly trustworthy. We tell Him our pain and allow Him to handle those who would hurt us. Read more: http://www.compellingtruth.org/pain-of-betrayal.html#ixzz2R9Mywb6F
they betray you out of jealousy or had motives in the first place but those people really aren't friends if they betray you or even try to i would keep your life stuff to yourself and never share things that are important to you and call them associates until they have treated and have been a. really good friend and a honest one at that to trust no one but family and the others well other people half to gain trust in order to be trusted and be in your life
It seems to me that people like to be close to people who are down and out or fell hard from a relationship Once that hardship is gone many times so is the so called friend. I know from experience. So the close ones to you become jealous when you do good. It's hard to find a true friend and if u do don't let them go.