I don't think it's about politeness. Politeness is about saying please and thank you, or holding a door open for someone. Or perhaps I am being impolite by not understanding that many people these days need a 'like' for whatever they do.
I can be genuinely happy for someone who is excited about getting a new car. We've all had that new car feeling at some point, but I don't think it warrants congratulations. As I said earlier, they should be reserved for achievements.