Because you have a sub personality that is suppressed and wants to say something but another sub personality won't let it.
There's a good exercise you can do to release this:
Get a clipboard and an A4 pad and two pens.
Put a line down the middle of the page.
With your main hand (i.e. if you are right handed) write a question on the right hand side of the page - something like which one of my sub personalities is making me grind my teeth. Then with your left hand write the first answer that comes to your head - no matter what it is - don't edit it just write it. If the answer is not what you want you need to keep asking it questions and proding the personality GENTLY like you would a child. Remember it has been suppressed for so long - it may be shy or you may get a few sub- personalities coming out at the same time. You'll find each sub-personality has different handwriting and usually a different way of communicating. Be patient with this method. It does work. You have many different sub- personalties - hundreds actually and whilst some of them work in your conscious mind, you are doing this to reach the suppressed ones. The ones who don't get a chance to say what you want.
Also by doing this it may trigger dreams or realisations on things that happened in your childhood regarding the questions you are asking, so be prepared for that also. This is a very good way of becoming a balanced person and letting suppressed emotions out. Do this as often as you want. A good book on this is Absolute Happiness by an Australian Michael Domenkyo Rowland. He was the one who turned Ferggies luck around!!