His smile is the sweetest thing ever, so it works-- but it was not the intended focus of the video. This grandfather was supposed to be capturing a man's proposal but instead captured his adorable genuine reaction when he did not realize that the camera was facing him the whole time.
According to the caption on the video, this is how it all went down:
"We had family visiting from out of town that wanted to check out the Sandia Peak Tramway in Albuquerque, NM. At the top was a man getting ready to propose to his longtime girlfriend of 8 years. He pulled me aside and handed over his iPhone so that I could capture the moment. My hands were full so I passed my phone and gimbal to grandpa. He thought he was filming the proposal the whole time but all we saw on playback was a genuine, heartfelt reaction to the proposal itself - he had the phone backwards! Awesome video. Once we realized what happened we all had a good laugh."
Hopefully someone else was able to capture the magic moment, but this is still a pretty great one to have as well!