[Infinity war spoiler stuff] (#s "You see the thing about Marvel is that they just love putting stuff in trailers and then changing them to subvert expectations. Like Gamora in the trailers walks hand in hand with Thanos as an adult, but in the movie she's a child. In the Civil War trailers Tony says to cap that they used to be friends in a sad voice, while in the movie he says it in a blinding rage.

Anyways the Thanos line there is to highlight how Thanos SHOULD feel at the end of the movie, but he doesn't smile at all. Chances are this line was brought up to point out to audiences that Thanos isn't happy despite having achieved his success, and this depression he seems to have over his daughters death may play a key role in the following movie.")