First of all, we should all be completely focused on the fact that we’re about to become one with God when preparing and going up for Communion. Hence, we should do everything we can to prevent distractions. Looking downwards in prayer is a great way. Thus, we don’t have to worry about people smiling.
Secondly, we have to distinguish some things. There is false happiness and joy, and there is true happiness and joy. To see an example of false happiness and joy, go to a Protestant service with their goofy hymns and shaking and the whole deal. To see true happiness and joy, watch how devout nuns live their lives. What we have to realize is solemnity. People are not “unhappy” if they aren’t smiling. I know several priests who, probably because of their temperament very rarely smile. Yet, they are very devout. Not to say the ones who smile are not devout, of course, but I’m just trying to show that not smiling does not mean unholy. So when people go up for communion, they have (or should have) a great interior joy that doesn’t necessarily culminate in a smile. I myself am naturally very serious and I naturally have a serious expression on my face most of the time. This does not mean that I hate the world. It just means that I’m naturally solemn and serious.
Actually, I have to say that the way people take pictures nowadays is not as good is it used to be. People put on these big smiles that somewhat make the picture look fake. I like how in older pictures, the people aren’t putting on a fake smile, but instead are looking normal. Not that people smiling is fake, but the picture is capturing something that’s been “planned” already. With the older pictures, they just took pictures of how the people looked.