As a thank you gift to Data, Q gives him the gift of laughter, but in fact, it's not the first time data is seen laughing. In Star Trek: The Next Generation: Manhunt
(1989) Wesley, Cmdr. Riker and Data are joking about Captain Picard's reaction to Deanna Troi's mother and Data is laughing along with the others. Although Data may have pantomimed laughing in the past, he states in Deja Q that he does not know what laughter "feels" like. This gift grants Data the knowledge of what it truly feels like to laugh. Not just Data approximating laughter the visual actions of laughter. See more
There's still this matter of the selfless act. Now, you and I both know that the Calamarain would have eventually destroyed the Enterprise to get to you. And that's really why you left, right?
It was a teeny bit selfless, wasn't it?
GRRR, yeah, and there is my problem! See, I can't go back to the Continuum and tell them that you committed a selfless act just before the end. If I do, there's gonna be questions, there's gonna be explanations for centuries!
Star Trek: The Next Generation Main Title
Composed by Jerry Goldsmith
and Alexander Courage See more