Baekhyun was awfully shy. Orphaned at the age of 6, he developed a slight anxiety disorder of selective mutism. As a kid he had always said that he prefers to listen, but in pure honesty he was just not listened to. 

He had a difficult childhood, with allergies varying from bees to rubber latex, he needed to be extra careful of what he ate, wear or touch. His asthma didn't help either - he couldn't do anything too rough to his body, that meant no 400 meter sprint. He also wasn't allowed to be around anything 'dirty' and that meant no puppy for his 8th birthday. 

Baekhyun never thought about these things, he says thinking about them gives him head aches.

Upon meeting an extrovert that goes by the name 'Park Chanyeol', his heart started to go "ba bum, ba bum", without the feeling of an astma attack.