He had a crush on her. She noticed he was really attached to her, but she thought it was just because they were close friends.

But now she was sure of it. Heechul gave her the confirmation. He confessed to him his feelings for her some months before but he didn't want to tell her because he thought it would just makes things between them awkward.

What she couldn't understand was why he would say something like that to Heechul. Everybody knew they were best friend so it would be a surprise if he would tell her what he said.

Heechul said he maybe did that hoping he would have said to her about his crush on her. 

Maybe he wanted to see her reaction before confessing. 

The thing was Taeyeon didn't have any reaction. It was all like before, like if she didn't know anything.

He confessed to Heechul he had feelings for her for years, even before they had the duet for SM ballad in 2014.

This explained many things, like wanting her to be his partner for the song in his new album, or he following her on stage during SMtown concerts trying to stand closer to her, or his eyes never leaving her.

She knew he was always watching her, and she noticed how his eyes followed her when she went on stage some days ago during the concert in Seoul. 

She noticed how he went in the second row of people on the stage to be closer to her.

Between them there was Sunny, so they weren't really close, but she knew it