Why think but not speak?

The human mind thinks openly day and day with thoughts of joy, sorrow, and change. Everyday your have at least 1,000 thoughts, but what are those thoughts of? Some you open up truthfully with, some you hide, some give opinions, some hold back other thoughts. Yet why? Why must we think yet be afraid to say what we think openly with the ones we know? Is it for fear of hurting them, is it for denial to yourself, is it because your afraid of yourself, is it just because you think what you think should stay in your head. Sometimes it is good to keep them, yet sometimes you wish you could tell someone something all though it would hurt them. Sometimes you try to tell them your thoughts and your conscious holds you back, sometimes they do not listen, or sometimes they laugh in your face at such thoughts. Believe to me that I do speak my mind a lot, but only of the good thoughts, at least I try. I hold some back due to the fear of hurting them. Sometimes I disagree with my thoughts, sometimes my conscious holds it back, sometimes they do not listen, and sometimes they laugh at me. These thoughts do not go away though. It is as if my mind is holding messages, both new and spam yet I do not open them so they continue to pile on. After awhile, there is no sense trying to read them. Thoughts are a gift, yet they are also a burden. If God allows you to have thoughts then why are they the way they are? Why can we not say it when we really want to? Why do our thoughts haunt us? Why do we think but not speak?