Unexpected is a synonym of Casual. Casual is a synonym of Informal. Informal is related to Seat. Seat is related to Single. 
Concern is a synonym of Headache. Stress can cause/lead to Headache. Stress is a type of Express. 
Unexpected <--Synonym--> Casual <--Synonym--> Informal <--RelatedTo--> Seat <--RelatedTo--> Single 
Concern <--Synonym--> Headache <--Causes-- Stress --IsA--> Express 