For changing Lookup to Master-Detail relationship, ensure that existing records are properly parented. Before changing Lookup to Master-Detail relationship, all the records should have lookup values.
If a field is created on Object ‘B’ with relation Lookup/Master-Detail to Object ‘A’, then Object ‘A’ is Parent/Master and Object ‘B’ is Child/Detail.
Converting a lookup to a master-detail-relationship changes the organization-wide default to Controlled by Parent and the sharing model is updated to public read/write.