On the February 18 episode of Mnet's 'The Music Interview', Jonghyun revealed that he was jealous of Taeyeon's singing abilities especially during their trainee days.  

He said, "Through Taeyeon's expressiveness, you feel that she experienced an incredible amount, felt love and went through separations.  However, that feeling existed even during Taeyeon's school and training days.  She expresses emotions very well, so I think I was jealous of her from a young age."

SEE ALSO: CEO Kim Tae Ho of BeLift Lab reveals how the members of ILLIT reacted to the plagiarism accusations

Taeyeon said in relation to his training days, "He was so good looking that I was very jealous."

Jonghyun continued, "We would be given grades and I would always be in second place while Taeyeon would be in first.  Rather than thinking I wanted to beat Taeyeon, I would think, 'I would like to be number one, too, sometime,' and would assess every weekend, but when I heard Taeyeon singing, I would just observe her a lot."