The following contains minor spoilers for Justice League. If you want to be unspoiled, go see the movie first, then come back.
The newest entry in the DC film universe opens today, and it wouldn't be a cinematic universe without references to other films that are in the franchise, especially the one released earlier this year, Wonder Woman. This includes a couple of references in the movie to Chris Pine's character of Steve Trevor, who sacrificed himself to save thousands of people. For what it's worth, it looks like Diana isn't entirely over it.
The first reference doesn't mention Steve Trevor by name, but he is clearly in Wonder Woman's mind. She's meeting with Cyborg for the first time and she's trying to convince him to join the team, and end his self-imposed exile. She talks about how she gave up on the world herself following the death of somebody special, and how she realizes now it was the wrong decision. The line also works as a call back to Batman v Superman: Dawn of Justice where Diana mentions her decision to isolate herself from the world of men.
Later on, the team is discussing the best course of action to take in dealing with the looming threat, and Wonder Woman and Bruce Wayne find themselves on opposite sides of the issue. Wayne goes after Diana for being MIA from the world for so long, when she could have been of great value. He takes her to task for only reappearing after Lex Luthor found a picture of her "boyfriend" Steve Trevor. Diana snaps a little bit at the use of Trevor's name against her, and gives Bruce Wayne a punch to the chest that knocks him across the room.
While Steve Trevor has long been dead by the events of Justice League, it's clear that the DC film universe doesn't want us forgetting him. The character made a significant impact on Wonder Woman as a person, and they want it to be clear that even now, she's being affected by him being in her life.
It's a nice nod to the Wonder Woman movie, which is important since the film is currently the highest grossing part of DC's film franchise. There's a better than average chance that the audience members of Justice League have seen Wonder Woman, and by bringing it up, it helps the audience connect better with the new movie as well.
If Steve Trevor's death is still causing Wonder Woman to punch people in the modern day, there's a good chance the wound will be even fresher in Wonder Woman 2, which is expected to take place someplace in time between the origin story movie and Justice League. Justice League is in theaters now.