I think there is a place for brutal honesty and a place for sensitivity...it really depends on the circumstances.

In your case, I don't think you were negative as it's your space and you don't need to reason why you don't want someone there...

However (and this is an EXTREME example)....if there is someone who is standing on the edge of the roof and going on about the economy...then NO, you don't agree with them and tell them it's a great idea...

It's just a case of using your best judgement in the situation. And yes..we all mess up trying to do our best, too. Nobody gets it right all of the time.