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).
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).