I think a better way of posing the question would be "When did the Watchtower lose it." "It" being the beginning of of the end for the Watchtower. I know when I saw it. Things started unraveling when the powers that be (Governing Body) started taking any joy out of being a Witness.
It started with little things. Conventions shortened. Food service stopped at Assemblies and Conventions. Circuit Assemblies went from two 2 day assemblies to one 2 day and one 1 day.
The next bit of joy that was removed were large gatherings. As a kid I loved the congregation picnics. The whole congregation would show up to a park and have a huge potluck cookout. Sometimes we would do bonfires on a beach. People would bring instruments and play music together.
I realize a lot of what I wrote was through the eyes of my child self. It just seems like there were good times. Then the Governing Body killed any joy that was left.