Kevin Jonas Ignores Fan's Photo Request ... In Saddest Video Ever

This video ... IS SOOOOOO SAD!!!!

A passionate Jonas Brothers fan waited outside Katsuya in Hollywood last night -- in the cold and the rain -- for a moment with her hero, Kevin Jonas ... only to learn what rejection feels like.

It was SOOOOOO SAD!!!!

FYI -- Kevin DID take pics with a bunch of fans as he left the restaurant with his wife ... trying to please everyone ... but the superfan got lost in the crush and never got a chance to pose with her hero.

Still, SOOOOO SAD!!!!