Please bare in mind that I am British, not American.
I was a child still attending school at the time it happened, most likely I was either studying or bullying neural typicals as was the status quo at our school.
However, I doubt I'll ever forget what happened when I got home that day.
There was no one to welcome me home, but there was a large commotion and a great deal of shouting coming from the garden.
It was celebratory party. There were a lot of family and friends of the family there that I had never met before, as well as some neighbors and businessmen in suits. It was an odd and very confusing day for a young autistic child, one that I didn't make sense of until many years later.
The reason why so many British people celebrate 9/11 with such fervor and gusto, is because it pretty much single-handedly killed the America's right to control it's own commercial and economic functions.
When savvy American Businessmen say that Britain is the sole reason America is in so much debt, they're no lying, and 9/11 is the reason why.
Before 9/11 American economical analysts predicted that the Dollar would be worth twice as much as the Pound. The exchange rate nowadays is ?1.00 = $1.56, and there was a time before 2008 when it was two Dollars to the Pound.