> Why is this true?

Because of something called the "Dependency Ratio": http://en.wikipedia.org/wiki/Dependency_ratio

The basic concept is that in order for there to be a high quality of life for most people in a society, the majority of them need to be capable of doing economically productive work. Otherwise you have a large number of dependents (juveniles and/or retirees) whose sustenance must somehow be garnered from a relatively small number of workers.

In China, for example, demographers talk about the "4:2:1 generation", which is the long-term effect of improving longevity combined with the one-child policy. This means that there could be four retired grandparents and two retired parents to every one working individual. Somehow, the social services for six people would have to be supported off the tax revenue from one. That's just not possible, and could very easily lead to a bad outcome for society as a whole.

(This isn't to discount the problems that come from overpopulation -- I'm just pointing out that it cuts both ways).

Too-rapid population growth can also lead to high dependency ratios -- in the form of children rather than the elderly -- and this also makes it difficult/impossible to support adequate social programmes. Historically, the best-performing economies tend to be those with very low dependency ratios, which is typically the result of a baby boom that is quickly followed by a baby bust. That produces a bulge of economically productive workers, the revenue from which can be spent on things like infrastructure and investment rather than social-support programmes which have less of an impact on GDP.

(Incidentally, this is a large part of the reason why China is going so bonkers with infrastructure development, building high-speed rail and ghost cities for which there is arguably no market. They're doing this because now is the only time they can. In 15 years, China as a whole will be substantially wealthier -- with enough people who are genuinely able to afford high-end condominiums and high-speed rail -- but at that point their youth bulge will be gone, scarcity of labour will drive up wages, most government revenue will have to go to healthcare and social services, and they'll have no ability build that kind of infrastructure. So they're building it while they can, well in advance of their actual need for it, which is actually pretty damn clever.)

What surprises me is that they consider low fertility rate a problem. There are plenty of people from other countries that would be willing to work in Western countries with low fertility rate.

If fertility rates are below replacement level, then let them be and let immigrants from a third world country work in their place. There are plenty of people in this world already.

> I guess the reason this quote bothers me so much is that increasing the population in a lot of countries would mean bringing new babies to a life of poverty and misery.

A lot of people would be surprised to learn that the impoverished peoples in a many those countries are a lot happier we expect. All-in-all they aren't much less happy than we are.

Not saying that wealth isn't a good thing, but it doesn't seem to be a major determining factor in happiness. Not unless you're too poor to get your 1,800 calories or you think you're entitled to wealth you don't have, anyway.

I am not refuting your point ("money does not happiness make") but AFAIK, it is a known fact that people tend to a state of average happiness no matter what their condition is (e.g. if they lack the ability to walk or if they own private jets).

But there is a correlation between the youth/child death rate and the number of kids, and it can be generally agreed death impairs the pursuit of happiness.

(Though, quite likely child death implies many kids, not the other way around)