A lot of the time, the people who are constantly critical of everything are usually either attention whores with some sort of hero complex, chicken little types who think the sky is falling when something they don't understand happens, or let their inferiority complex cloud their judgement and expect other to follow suit.
I think we need to draw a line between criticism and complaining, the difference is that criticism is designed to improve or change something whereas complaining is simply being reactionary to something, often because we don't care or don't want to know.