Why does DDD help define microservice boundaries?

Because a Bounded Context from DDD is a natural foundation for designing microservices.

The key points:

1. Each context has its own model and language, which matches the idea of an independent microservice.
2. Explicit interaction boundaries (through contracts, events, APIs) make it possible to build loosely coupled services.
3. No shared databases: each service manages its own aggregate and data.
4. Isolation of change: business rules inside one context can change without breaking the others.

Summary: DDD shows where a system's natural semantic boundaries lie, and therefore where microservices should end.