It depends on the extent of it, or how they are projecting themselves. Personally I like to try and act friendly toward people most of the time. There is no ulterior motive behind it - I just do not see the point in being the opposite, because I got bullied a lot many years ago so I dislike being mean-spirited due to knowing what it is like when you get mistreated. However, at the same time, I do not believe in acting happy and friendly all of the time - especially if you do not feel like it. If you are upset or in a "bad" mood I believe that you should be allowed to express it and should not be expected to put on a fake smile and such.
Now on the other hand if we are talking about individuals who act overly happy all of the time - then yes I do get suspicious of them. They are similar or sometimes the same as those who post "positive vibe" quotes on social media, but in reality are very rude. So again: it depends on the "type" of happy person that you mean. The genuine happy individuals, or those who are exaggerating and possibly false.