Not sure why we cannot make a custom lookup field a mandatory field, when the looking up object is a standard SObject.
e.g. Product Holder (Custom obj) has Component lookup field looking up Product2. But it seems we cannot make this lookup field mandatory, it gives below error when we try. Custom Field Definition Edit
Error: Invalid Data. Review all error messages below to correct your data. Cannot add a lookup relationship child with cascade or restrict options to Product2 (Related field: Domain)
But we could make it mandatory on layout/or using a validation rule or trigger. Any reason why we cannot do this on sObject itself? Thanks.