Crowded is the opposite of Alone. Alone is related to Leave. Leave is etymologically related to Love. 
Reply is related to Question. Question is related to Doubt. Doubt is a type of Uncertainty. Worry is created by Uncertainty. 
Crowded <--Antonym--> Alone <--RelatedTo--> Leave --EtymologicallyRelatedTo--> Love 
Reply <--RelatedTo--> Question <--RelatedTo--> Doubt --IsA--> Uncertainty <--CreatedBy-- Worry 