Chip Smith points to a study, published in the American Journal of Sociology in 1932, with the surprising result that 30% of a broad sample of children studied expressed a wish never to have been born. (I know someone pointed me to this before, but I forget who it was.)
Life's cheerleaders will no doubt argue that such wishes, while common, are most likely fleeting and not of a serious nature. However, I think this study must suggest to even the cheeriest of us that most people's feelings toward life are ambivalent from the very beginning of mature consciousness. A feeling of certainty that anyone brought into being will be grateful to his creators is not justified. The essential value of one's own life is not a feeling universally shared.
Many, many people are not glad to be alive. They are among the most seriously wronged by being brought into existence. But (and the author of the above study is a case in point) their position is pathologized and not taken seriously; even though cheeriness is not the universal position, it is assumed to be the correct position. Any deviation from gratitude for life does not, from the dominant point of view, need to be sincerely considered.