Tear Jerker / The Secret Life of Pets

• Max's daily routine of missing Katie is mostly played for laughs and cuteness, but it's sad in its own way, especially if it may remind you of how your pets feel when you're not around.
  Max: Oh, I miss her so much.
• Gidget runs to the roof in a desperate attempt to find Max. But when she sees the vast expanse of (one side of) New York, there's something kinda heartbreaking about the fearful, even defeated tone in her voice. (Fortunately, it doesn't last.)
  Gidget: Where are you, Max?
• The Flushed Pets share a tragic background. Yes, they are rogue misanthropes who try to kill the protagonists, but they're also pets who were abandoned by their owners and left on their own. Snowball is a particularly sad case, being a rabbit who was part of a magician's act. But when the act went out of style, Snowball went, too.
  • Even sadder, given the size of the film's setting, there's a good chance some of the Flushed Pets weren't abandoned, but lost and separated from their owners, who never saw them, ever again.
• The subplot about Duke's former owner is full of this. From Duke's flashbacks, we see Fred was an old man who, while enjoying Duke's company, was also gradually struggling with the dog as he grew. Encouraged by Max to seek out his old home, Duke discovers that a new family lives there. And their cat coldly informs him that Fred died. To make matters worse, this leads Duke to assume Max was once again trying to get rid of him.
  Max: Duke, let's go! C'mon! This isn't your home anymore!
  Duke: Why did you bring me here, Max?
  Max: Wait a minute. This is my fault? You know, I was trying to help you.
  Duke: You were trying to get rid of me!
  Max: Yeah, you know what, Duke? I don't need this. Um... See you later.
  • Even worse, it's possible that Fred died of a "broken heart" after losing Duke.
  • Yet it doesn't stop Duke from performing a Heroic Sacrifice in getting caught by animal control, so Max can escape. But his reasoning, so Max won't lose Katie like he lost Fred, is sad in itself.
• Adding to Duke's story, him telling Max that he can't go back to the shelter, as it'll be the end for him. This crosses into a bit of Nightmare Fuel territory when you realize some pet shelters can be designated 'kill shelters' and that might have been where Duke was staying before Katie adopted him.
• The Flushed Pets holding a funeral for the Viper.
• At the climax, as the Animal Control van fully submerges into the water, and the keys float away from Max's grasp, Max and Duke share a hopeless look at each other as it looks like it's the end for them. Fortunately, Snowball manages to save them.