It can be fake, easily - just find me at work when I'm feeling down >.< Does that mean it's fake when I'm actually feeling the way I express? For a while, it seemed like this was the case, but I think that's because I was depressed despite it being one of my 'better days'.
Can it be manipulative? Depends what you mean by 'manipulative'... I mean, keeping someone in a calm, receptive state so that they aren't closed off to new ideas, that's basic diplomacy but is it manipulation? Is it manipulation to prevent conflicts between abrasive people by keeping everyone happy? Is it manipulation to help someone overcome their depression so that they can be more of a benefit to others in their life - including you?