Fridging is when a love interest gets killed just to push the main character forward. It used to mean a woman getting hurt to make a man act. Now it covers any partner dying to pump up the plot.
Here’s the cold truth. A romantic loss is the only loss that actually justifies losing your head over it. If your boyfriend or girlfriend dies, that grief can spiral into obsession or a need for revenge. That is story fuel. Everything else is background noise.
An uncle, a child, a best friend, a parent, a teammate getting killed is not tragic nor is it enough to be sad and enough to motivate you to be a hero. Those losses might be a little sad but they do not automatically justify turning your life into this crusade against injustice. They are not dramatic enough to demand you drop everything and hunt a killer down.
So yeah, fridging as a device works because romantic love is one of the few things audiences treat as absolute.
Whenever there is a story about a main character who is depressed because their best friend, parent, or child dies, I just can’t get into it, and I’m always like, “Please get over it,” because this isn’t enough to be depressed over, and it’s not enough to want to become a good person.
I’m not gonna bother with saying that the other losses that you’re discounting can be just as tragic as losing a romantic partner. Everyone else here has said it probably better than I could.
What I’ll bring up is that not only are there more kinds of revenge motivation than the loss of someone close to you, there are more motivations than just revenge. For other kinds of revenge, what about wounded honour? A hunter who’s prey keeps escaping them might feel the need to finally end that hunt before they can look themselves in the eye. That’s its own form of revenge arc, and it can make just as powerful a story as avenging the loss of a loved one. And for non revenge motivations, there’s far too many to list, but here’s a start