Everybody understand that there is no everyhting bad or everything good - nothing can last long. So, when everything is good in peoples life, they still expect that something bad may happen. This expectations provide the negative way of thinking. But our human nature is a such thing, that when we have no problems and everything is bad, we expect much more troubles, than we already have. And those people, who think positively all the time, survive all their troubles much more painfully, because they dont expect this.