The urge to grow appears to be built into all of us. This amounts to an increase in our impact on the world, either in terms of what we consume or what others can identify with us (such as artifacts we create or offspring that carry our DNA).
Growth is typically either linear or exponential. If each day we acquire one new thing that is equivalent to each of the things we already possess, then our growth is linear. If, instead of one new thing, we acquire a constant fraction of what we had the day before, then our growth is exponential.
It would appear from history that people prefer exponential growth over linear growth; yet linear growth is more likely to be sustainable, whereas exponential growth is always and inevitably unsustainable.
One reason we grow is to offset potential losses. Throughout human history, populations have been subject to losses due to a variety of causes that include disease, predation by all species (including our own), and lack of food. To maintain a constant population, we must increase the population by the amount of the losses. Increasing a population beyond its losses requires an increase in resources. We also have the option of reducing the losses by disabling the mechanisms responsible for them.
Another reason we grow may be a prevalence of what I call relative thinking. Relative thinking is the tendency to define what we have as equivalent in value to what we’ve had before. For example, I perceive the person I am today as the same person I was as a child, even though by all objective standards we are radically different. Each year that we “improve ourselves,” the “new” us becomes the standard that needs improvement. Exponential growth follows from this when we conflate value (or quality) with quantity: we want to grow by the same amount, as a fraction of what we have now.