Why is Singleton often called an "anti-pattern"?

Singleton is often considered an anti-pattern because of its side effects on scalability and testing:

#Global state

Singleton turns into a "hidden global variable." This makes it hard to track who changes the object's state and where. It breaks the isolation of components.

#Testing difficulties

It is hard to replace with mocks because the instance is fixed. In unit tests you have to reset the state manually or use reflection.

#Violation of the single responsibility principle

The class solves a business task and manages its own lifecycle at the same time, which complicates the architecture.

#Hidden dependencies

Code that uses Singleton becomes implicitly dependent on it, which hurts readability and reuse.

Conclusion: Singleton is useful for limited tasks (such as loggers), but overusing it leads to implicit dependencies, hard testing, and a fragile architecture.