I have developed a similar habit over time. I am often the "smart guy" in my social enviroment (I'm not particularly brilliant, but neither is my usual enviroment), and I can often identify major flaws in other people's reasoning. Despite this, I very rarely point them directly out. Social conventions usually state that this behaviour is considered unpolite, indirectly implying that the other person is dumb. It can be even worse if the other person is emotionally attached to the thought. So, unless I am discussing with a very close friend, I usually restrain from making meaningful comments.