Facade and Builder Pattern behind the scenes

I've done an exercise with Facade and Builder Pattern combined. Root is a simple LoginService-Interface

public interface LoginService {
    void addUser(int id, String username, String password, String mail);
    boolean login(String username, String password);
    void logout();
}

Other classes are a LoginServiceDecorator and some concrete Decorators. Finally a Builder, who is tested this way:

service = new LoginServiceBuilder().withValidation().withEncoding().withLogging().toService();

with some test cases.

All was fine, until the implementation of the toService()-method, where I had no real good idea how to implement. I show were I stucked:

LoginServiceBuilder

public class LoginServiceBuilder {
/*
 * Note that the decorators must be connected in reverse order of the
 * builder method invocations. So we use a stack to hold all decorator
 * instances and at the end of the building process, we connect the
 * decorators in the right order.
 */
private Stack<LoginServiceDecorator> stack = new Stack<LoginServiceDecorator>();

public LoginServiceBuilder() {
}

public LoginServiceBuilder withValidation() {
    stack.push(new ValidationDecorator(new LoginServiceImpl()));
    return this;
}

public LoginServiceBuilder withEncoding() {
    stack.push(new EncodingDecorator(new LoginServiceImpl()));
    return this;
}

public LoginServiceBuilder withLogging() {
    stack.push(new LoggingDecorator(new LoginServiceImpl()));
    return this;
}

public LoginService toService() {
    // Here I stucked
}

Well, finally I gave up, and took a look at the solution:

public LoginService toService() {
    LoginService service = new LoginServiceImpl();
    while (!stack.isEmpty()) {
        LoginServiceDecorator decorator = stack.pop();
        decorator.setService(service);  // just to set the private member there (Type of the Interface as shown at beginning)
        service = decorator;
    }
    return service;
}

Why I'm still scratching my neck is following: For me it looks like when the Stack is empty, the service is simple the last one it caught. Maybe someone could explain me with soft words, why I should have all decorators now.

Thanks much in advance