Adult hippocampal neurogenesis, i.e. the formation of new neurons within the existing neuronal network of the dentate gyrus, is subject to modulation by internal and external factors. Among them, voluntary physical exercise is one of the best investigated positive stimulators of neurogenesis in laboratory rodents. Straightforward translation of the observed running-increased neurogenesis has nourished the commonsensical idea that physical challenges would keep our brains healthy and young. However, several lines of evidence indicate that the tempting assumption that, through physical exercise neurogenesis increases and hence cognition improves, might fall short of more complex effects and interactions. In the present work we argue that motivation may be a key factor in determining whether exercise will positively affect neurogenesis in the hippocampus of laboratory animals. In addition, it has been shown that in genetically heterogeneous wild mouse species hippocampal neurogenesis can be exercise- and context-independent. It appears that in wild rodents adult hippocampal neurogenesis is stabilized to the sum of transient pleasant and aversive stimuli characterizing a natural environment. Variability in the regulation of adult neurogenesis questions if the concept of an improved cognition mediated by a running-increased neurogenesis can be easily transferred to the human condition. The present review surveys the research on exercise and hippocampal neurogenesis in the context of motivation, genetic background and species differences.