what is common sense?

Asalaamu alaikum!

Common sense is not so common at times. Often we try to make the smartest decisions but it ends up being the most dumbest one. After the mistake has been done we start to wonder, "what the hell was I thinking?".  I pretty much had this moment yesterday over a craigslist ad. Its too soon to speak about because I'm still emotional about it but…. What the hell was I thinking?!