The first time this concept was brought to me like a huge WHAM in front of my face was a few years ago. Recently a friend of mine is learning this concept herself. She found a closed door and she's trusting God and thanking Him for this closed door. She knows that God knows what's best for her, even though it's difficult to understand why.
Her email reminded me of the value of closed doors. How many times are we grateful for closed doors? Probably MUCH less often than our gratefulness towards open doors, since we rarely understand the value of closed doors. Even if we do come to that understanding, it usually comes much later.
I remember that after September 11th, there were many TV shows concerning that incident and I remember that some of them couldn't go to work on time due to some problems (a flat tyre, a sick child). That day they would have probably cursed the "problems" they had that made them unable to go to work on time, but I can imagine their gratefulness once they found out that it was actually a blessing in disguise.
I realize that sometimes we try SO hard to open doors ourselves and sometimes we even try to open one particular closed door over and over again with all our might without realizing that it might be best to leave it there and then.
But anyway, we live, we learn. Mistakes don't make us. We are not our mistakes. :-))))