I doubt that the sneezing has anything to do with being happy. Sneezing is cats is for the same reasons as it is in humans. Irritation, blockage, allergic reaction, or sinus infection. If there are no other symptoms (eye or nose draining, wheezing, difficulty breathing), it is most likely due to some temporary irritation or blockage such as inhaling a bit of food or litter dust, or even from playing with a feathery or furry toy.