I know what that's all about. I have a co-worker who is all syruppy all the time and it makes people sick in their stomachs to be around her as they know that it is not genuine. [That's other people's reaction, as I simply try to see the good that she is attempting to achieve.]
Responding as a cycnical person I can say that one must know how to fake it. One must give it 100%. For instance, take the smile that a person greets you with. If it is one of those instant smiles it does not have much impact. If you however, delay your smile for a fraction of a second and gradually let your smile develop it is perceived as warmer, more genuine!!

Your body must also be more open/welcoming as well too!! So perhaps the ones that are preceived as "fakes" just don't know how to do it right.
Now if you cannot fake it....People often get confused about "it's good to be kind to people and even make them feel good about themselves" idea. We can always be kind to people by being truthful/genuine in the best possible way that we can. When a instructor/dancer/co-worker is not truthful it often shows up as differences in body language from facial expressions and what is being said.
My advice is if you cannot fake it then be genuine 100% of the time. A person feels better if they are interacting with you on a genuine level, rather then getting syruppy/flirty stuff which they realize is supposed to make them feel good, but isn't as they can tell it's fake. It's as if a person is trying "to put one over you"!!! Not good.
I know that I have given an either and or scenario and life is not just black and white. There are many circumstances/situations that are a shade of gray.