Using disposed observer does not re-subscribe to the source

I'm trying to reuse an Observer for a Single and Observable stream by creating only a single instance of DisposableSingleObserver/DisposableObserver and passing them through it through the subscribeWith() method on the stream like below:

public class SomeClass {
    private DisposableSingleObserver<Object> observer;

    public SomeClass() {
        observer = new DisposableSingleObserver<Object>() {
            @Override
            public void onSuccess(Object object) {
                ...
            }

            @Override
            public void onError(Throwable throwable) {
                ...
            }
        };
    }

    public void doSomeStuff() {
        singleStream.subscribeOn(...)
            .observeOn(...)
            .subscribeWith(observer);
    }
}

The above code resulted in ProtocolViolationException when I tried to subscribe with the single observer instance multiple times with message:

io.reactivex.exceptions.ProtocolViolationException: It is not allowed to subscribe with a(n) com.package.name.SomeClass$1 multiple times. Please create a fresh instance of com.package.name.SomeClass$1 and subscribe that to the target source instead.

So I modified the code a bit like below:

public class SomeClass {
    ...

    public void doSomeStuff() {
        if (observer != null) {
            observer.dispose();
        }

        singleStream.subscribeOn(...)
            .observerOn(...)
            .subscribeWith(observer);
    }
}

When I executed the above code, the ProtocolViolationException was no longer thrown and I was able to get event from the single stream successfully. But when I tried to call doSomeStuff() method couple of time, it completed successfully on first call but did not emit any event on second call. I was able to confirm that the subscription happened through doOnSubscribe() both times but the code emitting event for single stream was never executed on second call. So I have three questions:

1. Why was the ProtocolViolationException thrown in the first place?
2. Why was I able to get the event on first subscription but not on the second subscription even though we subscribed to the source in both cases?
3. How can I reuse observer by using only a single instance?