Cooper named the barrel maker—the craftsperson who built and repaired wooden casks, tubs, and vats that held beer, wine, water, and trade goods. Middle English couper/cowper points to that specialist trade; before metal cans and plastic, coopers were essential European infrastructure.
The surname is therefore occupational and polygenic: many towns had coopers, so many unrelated Cooper lines exist. Dutch Kuiper (“cooper”) sometimes merged into Cooper in American records. It is not related to “cooperating” as a personality trait. Reunion gloss: staves, hoops, and watertight barrels.