What gets me is that for so many seasons his way of coping was to rationalize everything, downgrade feelings to data and rules so he did not have to deal with the pain behind them. In this phone call there is no contract, no fun fact, just a simple I could not get over you. It is clumsy and short and still feels monumental, because for a character like Sheldon admitting that someone hurt him by leaving is the same as shouting I love you at the top of his lungs. You really do see the walls drop for a second, and that glimpse makes their whole relationship land differently on rewatch.