Headcanon: Flash has the gift of making Sunset laugh to the point where she can’t breathe. She just finds it hilarious when he acts like a total dork. To be fair, it has a lot to do with the fact that’s she in love with him. And Flash thinks Sunset’s laugh is freaking adorable so he’s glad he can make her laugh so hard.
 
The concept of this is Flash is singing in a funny voice purely to amuse Sunset and he’s doing a really good job because Sunset is laughing her head off. I mean, maybe it’s just me but if you’re in love/attracted to someone, it seems like you end up thinking they’re funnier than most other people would. I speak from experience.
 
My Little Pony: Equestria Girls is (C) Hasbro Studios