Sad face
We do not understand why someone would react to such a nice comment with a sad face. Only if this person was very creepy or a stranger that they did not want to talk to. If that were the case then a sad face or a cold response would be appropriate and understandable.
But if they were just not interested they could have always just not replied, sometimes that’s for the best. This guy seemed to really pour his heart out to this girl, we hope he is doing okay after such a rejection. Maybe he could try this same line on some other girls.