There are many reasons as to why you may want to convert a Lookup to a Master-Detail Relationship. You may need to roll-up data, simplify security settings, or maybe you had initially meant to create a Master-Detail but instead created Lookup and are now just getting around to making the change.
- The current Lookup field on the Child object must be populated in every single record. If there are records without this field populated, the conversion will not occur.
For Example – Object A (Parent) and Object B (Child) has lookup relationship. Before you convert the lookup relation to master detail relation make sure child (Object B) object record has Object A lookup field populated for all records.