How would you decide between a master-detail relationship and a lookup relationship in a Salesforce data model?

I treat it as four separate questions, and any one of them can force the answer:

1. Sharing: Does the child need to inherit the parent's sharing and ownership? Only master-detail does that. If the child must have its own owner or its own sharing rules, master-detail is out immediately.
2. Roll-ups: Do I need roll-up summary fields on the parent? Those are native to master-detail only. With a lookup I would need Flow, Apex, or a tool like Rollup Helper, which is extra code to maintain.
3. Lifecycle: Should deleting the parent cascade-delete the children? Master-detail does; a lookup can be configured to clear the field or block the delete instead.
4. Write volume: This is the one people forget. Every DML on a master-detail child locks the parent. On a high-throughput transactional child that produces row-lock contention, so I will often deliberately choose a lookup plus an asynchronous roll-up, accepting eventual consistency to protect write throughput.
5. Practical note: Master-detail also caps at three levels of nesting and the child inherits the parent's record-level security, which can surprise people during a security review. I document that trade-off explicitly rather than discovering it in UAT.