you would Think because Rational. Rational is related to Reason. 
Grand is similar to Important. Important is related to Necessary. Necessary is the opposite of Casual. Casual is related to Remark. 
Think --MotivatedByGoal--> Rational <--RelatedTo--> Reason 
Grand <--SimilarTo--> Important <--RelatedTo--> Necessary <--Antonym--> Casual <--RelatedTo--> Remark 