When you make decisions, you like to believe you made them for a reason. This is particularly true of investing. I bought this stock for rational reasons, right?
We recommend watching this TED Talk by Petter Johansson, which discusses Choice Blindness. His experiments explore that our rationale for the decisions we make might not be as strong as we would like to believe, and that we can actually end up justifying opinions that we didn't even make (as you will see in the video).
“So what this [first] experiment shows is, OK, so if we fail to detect that our choices have been changed, we will immediately start to explain them in another way. And what we also found is that the participants often come to prefer the alternative, that they were led to believe they liked.”
“So first of all, a lot of what we call self-knowledge is actually self-interpretation. So I see myself make a choice, and then when I’m asked why, I just try to make as much sense of it as possible when I make an explanation. But we do this so quickly and with such ease that we think we actually know the answer when we answer why. And as it is an interpretation, of course we sometimes make mistakes. The same way we make mistakes when we try to understand other people. So beware when you ask people the question “why” because what may happen is that, if you asked them, “So why do you support this issue?” “Why do you stay in this job or this relationship?” — what may happen when you ask why is that you actually create an attitude that wasn’t there before you asked the question.”
Of course, when you're investing, you need to remember you are fallible. That's why we here believe 1) You have to be very careful how you frame questions when learning about something from someone and 2) document your own decisions and reflect on your answers later, and 3) use a checklist whenever possible (see Checklist Manifesto).
“The first principle is that you must not fool yourself and you are the easiest person to fool.”