I’ve seen this happen over and over: code turns into a mess simply because we don’t extract logic that’s used in multiple places. It’s not about complex architecture or big design mistakes—just the small habit of directly calling functions like .Add() or .Remove() instead of wrapping them properly.

Take a simple case: a service that tracks activeObjects in a dictionary. Objects are added when they’re created or restored, and removed when they’re destroyed or manually removed. Initially, the event handlers just call activeObjects.Add(obj) and activeObjects.Remove(obj), and it works fine.

Then comes a new requirement: log an error if something is added twice or removed when it’s not tracked. Now every handler needs to check before modifying activeObjects:

void OnObjectCreated(CreatedArgs args) {
    var obj = args.Object;
    if (!activeObjects.Add(obj)) 
        LogWarning("Already tracked!");
}

void OnObjectRestored(RestoredArgs args) {
    var obj = args.Object;
    if (!activeObjects.Add(obj)) 
        LogWarning("Already tracked!");
}

At this point, we’ve scattered the same logic across multiple places. The conditions, logging, and data manipulation are all mixed into the event handlers instead of being handled where they actually belong.

A simple fix? Just move that logic inside the service itself:

void Track(Object obj) { 
    if (!activeObjects.Add(obj)) 
        LogWarning("Already tracked!");
}

void OnObjectCreated(CreatedArgs args) => Track(args.Object);
void OnObjectRestored(RestoredArgs args) => Track(args.Object);

Now the event handlers are clean, and all the tracking rules are in one place. No duplication, no hunting through multiple functions to figure out what happens when an object is added or removed.

It doesn't take much effort to imagine that this logic gets extended any further (e.g.: constraint to add conditionally).

I don’t get why this is so often overlooked. It’s not a complicated refactor, just a small habit that keeps things maintainable. But it keeps getting overlooked. Why do we keep doing this?