Sing is annoyed because his friend, you, caught often pressing likes on Wonbin's story instagram, then Sing sent an instagram message to Wonbin's account with this message:

"no need to pretend to be good looking, dude." "that makes her always likes your instagram stories."

That message made Sing's feelings for you even more obvious. Even though you doesn't necessarily press likes for Wonbin's Instagram story because you think Wonbin is good looking, but Sing is already emotional, this is like a sweet high school teenage romance story. Wonbin was the first to suspect Sing's feelings because of this message from Sing.