Back to the little cactus in my back yard. Given its vast expenditure on reproduction, what are its chances of surviving? In a word: diminished. What is spent on reproduction is not available for maintenance.
Switch now to humans. What is the signal that turns the organism's resources toward reproduction over maintenance? Insulin. When food is plentiful, insulin levels are high and this is a signal to the billions of uninterested cells (they don't care about your motives and goals) in your body to shift resources away from their own survival to the survival of the genes.
