If you don't and you basically lie about how you feel you're fake. Then again fake people make up most of the world and politics. So, if you're looking for pathetic orthodoxy go for it. Honestly, your thoughts are your true being. I personally believe that is what makes you as a person and if you don't express that who are you exactly?
Always saying how you feel helps you get over things easily such as overcoming speeches. Not only that but it feels good. Once you have cleared your mind out of things that you wanted to say a long time ago to someone, its relieving! Also, if you have told them what you wanted, you can both arrange something to fix this issue.
Imagine this: your friend never turned up to meet you for a burger as you arranged. You are really annoyed- how could they dump you like that? You turn up at their house angry and as they open door you tell them exactly what you think of them. Your friend looks at you teary eyed and slams the door. A few weeks later you find out that the reason they couldn't make it was because of a death in the family. You feel terrible because you said exactly how you felt without any attempt at understanding the context of the situation.
Saying exactly how we feel all the time is not conducive because it focuses solely on ourselves without considering the situations of others. Speaking out emotionally when one does not posses all the facts causes more harm than good and can put a strain on credibility, relationships as well as compassion and emotional maturity.