I just read the script for Valentine's Day... (Spoilers).

First off, this isn't the final script (for example, in the script Bradley Cooper's character is an African American).

That being said...

Cooper and Dane's characters have only scene together, at the end after Dane's character comes out at a press conference. All they do is hug. So that's why them being a couple isn't in the trailer.

Again, this wasn't the final script; it's possible they do more in the final version.