For most of human history, "more" and "better" have been pretty much the same when it comes to the things we want. Even today, a very large number of people live in poverty, and their main priority is more -- more food, more clothing, more medical care, more things. For them, "more" would still be "better."
But, for many of us, we have long ago passed the point where "more" is the same as "better." Every study that has looked at the correlation between wealth and happiness finds the same thing. Up to a certain point, more money make people happier. After a certain point, however, more money stops making us happier. Many of us are long past that point. McKibben starts with this observation, but then he moves further.
According to McKibben, our wealthy modern lifestyle is actually starting to make us less happy. We are social creatures, and living alone in massive houses, traveling in separate cars and the other things money tends to buy these days tend to isolate us from other people. This makes us less happy, in the end, not more.
And, finally, our lifestyle is less and less sustainable. Our food supply, for example, is highly dependent on cheap oil. While this has worked for a while, it cannot work forever. The demand for oil -- and other limited resources -- will grow spectacularly as some of those in poverty start to adopt some of our way of life. And that is so, even if population stops growing.
I found this book deeply disturbing, but I think McKibben is right about the problems he identifies. McKibben, however, is not so pessimistic. He thinks there are solutions that will allow us to live even happier lives by consuming less, not more. I sincerely hope that he is right, and that more people at least listen to what he has to say.