First encounter with Monty Hall problem. My roommate in college told me about it, and I was absolutely unable to see how it could be anything other than a 50/50

First encounter with Monty Hall problem. My roommate in college told me about it, and I was absolutely unable to see how it could be anything other than a 50/50 chance for the remaining two doors. It took me actually drawing out a grid of the possible scenarios to see and accept that he was correct.