you would Thank because Grateful. Thank requires Find. Person wants (to) Find. Person is the opposite of Group. 
Custom is related to Standard. Standard is etymologically related to Hard. Hard is related to Feeling. 
Grateful <--MotivatedByGoal-- Thank --HasPrerequisite--> Find <--Desires-- Person <--Antonym--> Group 
Custom <--RelatedTo--> Standard --EtymologicallyRelatedTo--> Hard <--RelatedTo--> Feeling 