angular 2 NgIf hides ever, even observable will get from null to object

Question

I would like to show the something only if the user is logged in. The user is an observable and to show/hide I would like to use NgIf.

But the NgIf hides the content forever, even I see could verify the user changed from null to an object

userObserver: Observer<any>;
user: Observable<any>;

constructor() {
    this.user = new Observable((observer: Observer<string>) => {
        this.userObserver = observer;
    });

    this.user.subscribe((x) => {
        console.log('next', x);
    });
}

public login(credentials: ICredentials): Observable<any> {
    this.userObserver.next(credentials);
    return this.user;
}

In the template I use the observable like:

{{user | async | json}}                       // will change
<div *ngIf="user | async">user here</div>     // will never show up

Edit I got it working using:

isLoginSubject = new BehaviorSubject<any>(null);
user: Observable<any> = this.isLoginSubject.asObservable();

But then we can change the question to why? what difference makes it out?