Professor Fred Gage explains that neurogenesis is an unstable process and is highly regulated by the environment.
One of the amazing things about neurogenesis is that it is not stable. It is highly regulated, so experience, behavioral experience, can affect the amount of neurogenesis that occurs. This can be divided into at least two categories. One is your physical interaction with your environment, so the physical motion can induce a proliferation of cells in the hippocampus, in the adult hippocampus. It looks like experiencing the environment, learning, acquiring information about the environment, also has some effect on survival. So, physical movement within your environment and acquisition, or learning, of information about your environment both have some effect on both the proliferation and the survival of these newly born cells in the brain - in the hippocampus in particular.