Why do living forms develop? Development, like evolution and culture, is a process that creates complexity by accumulating change. At any moment, the developing agent is a product of all previous developments, and any new change begins with and must build on those previous developments. Biological systems that are flexibly smart have relatively long periods of immaturity. Why is this? This talk will consider answers to this question using evidence from the first two years of life of human infants. The core ideas are that an adaptive system that can succeed in varied and novel contexts is slow does not settle too fast; develops new mechanisms of change and learning processes over the life time; develops in a series of different environments.