The structure of the trailer, which seems to be a "we do not have a lot of footage ready to show" trailer, is brief clips from Pixar films illustrating the emotions of joy, fear, disgust, anger, and sadness.
When they get to sadness, the viewer (well, this viewer) flinches in defense, given the many Pixar moments that have so effectively trafficked in the emotion. We are ready for something that will make us cry. Then we get:
Dory, from Finding Nemo, sadly asking Marlin if he likes her - but in a mocking kind of way laced with humor. Not really a sad moment.
The Finding Nemo sharks crying in exaggerated fashion - not sad.
The kid ants from A Bug's Life wailing - a goofy, not sad moment.
And your guard is relaxed.
Then comes an moment from Monster's Inc. where Boo reaches for Sully with tears in her eyes and he gathers her into his arms with a look of real pain on his face - a truly sad moment. And because they have lulled you into a false sense of security, that moment hits harder than it would have otherwise.
Then we get:
A real human moment of the Queen from Brave breaking down.
Woody saying goodbye.
Wall-E grasping his own hand in a gesture of unfulfilled need of human connection.
And, of course, Carl looking at Elle's empty chair.
These truly sad moments would have worked without the goofier few that kicked things off. But with them they just crush the viewer, and really drive hone just how good Pixar can be at eliciting emotion.
This is good marketing.