The appeal of a smile has to do with both aesthetics and perception. When people seek cosmetic dental treatment because they are unhappy with their smiles, chances are they have a legitimate problem. Some individuals' feel they show too much gum tissue or their gums are too prominent when they smile. Either their teeth appear too small, or so much gum tissue shows that the teeth are not making an impact. And a gummy smile can make a person feel self-conscious.
So what is the definition of a gummy smile? That's a matter of perception, and therefore will vary from person to person. But it has been shown that a smile will usually be perceived as gummy when four millimeters — a tad more than an eighth of an inch — of gum tissue shows.