The answer to this can be found in Chapter 6 of Guns, Germs, and Steel. There, Diamond presents and dispels what he calls misconceptions about how people moved from hunting-gathering to farming. Basically, Diamond says that we tend to think that people actively chose to move from one lifestyle to the other. This is not, he says, how it really happened.
First of all, Diamond argues, there is not a clear dividing line between these two ways of life. He argues that there are people who are sedentary and who are hunter-gatherers. There are also people who are nomadic but who engage in agriculture. There are people who are hunter-gatherers but who do things like clearing forested areas of unwanted trees to let the trees that they like thrive. Finally, there are people who have some agriculture and who also hunt and gather. Thus, there are not two ways of life that are clearly separate and distinct.
Second, Diamond argues that people typically did not actually consciously choose to become farmers. This was particularly true in areas where no farming had ever existed. On page 105 in the paperback edition of the book, Diamond argues that people in such areas
...could obviously not have been making a conscious choice or consciously striving toward farming as a goal, because they had never seen farming and had no way of knowing what it would be like.
Instead, the transition to farming was something that took place gradually, through a series of decisions and actions.
The transition to farming, then, was not an abrupt and conscious decision between two ways of life the way we think it was. Instead, it was a gradual change that would have occurred largely unconsciously.