What makes Tyrell so sad about his memories of the day he and Tariq threw water balloons off the top of Tyrell's building?

What makes Tyrell so sad about his memories of the day he and Tariq threw water balloons off the top of Tyrell's building is that Tariq said something that had the two of them laughing hard for ages, but Tyrell could not remember what Tariq had said that was so funny.