Um?
I agree that the gunshot was a little...unexplained. It was what would be termed "improbable". In writing and in movies, animations, etc, there are two things for the story: the impossible, and the improbable. The "impossible" is something you can do in the story, like having someone with superhuman powers, or an x-ray vision scope on a gun, et cetra--provided you've given the background for it, placed the viewer in the world that such a thing is possible in. The improbable is total randomality. The gun shot, in this case, was improbable. There was nothing to provoke it. However, this improbability is the basis of some really good comedy, such as ATHF, Sealab, and this flash. But I feel that in this case, the timing was wrong, and that is was made it not quite as funny as it could have been. Like, the kissing part, the song, was lovely in that it was a parody. It was cliched and purposefully so, which made it all the better. But then the gunshot was all serious. Like, no over-the-top agony from the man, the woman was all serious and angry with the shooting. It just didn't work for me.
As for the animation, I again agree with previous posts. It was very...refreshing? I think is the word I want. I enjoyed it. Especially when the hearts started in, those in particular I feel were well done. Keep at it, I really hope to see more from you!