Explanation
A validation rule on the Opportunity object is the best way to enforce the requirement that the Reason Lost field is required when the stage is changed to Closed Lost. A validation rule can specify a condition and an error message to display when the condition is not met. For example, the condition could be AND(ISPICKVAL(StageName, "Closed Lost"), ISBLANK(Reason_Lost__c)) and the error message could be "Please enter a reason for losing the opportunity." Making the field required on the page layout or universally required would not work, because the field should only be required when the stage is changed to Closed Lost, not for other stages.