This is a common condition in which the gums are seen when a person smiles. It can occur due to hyperactive mouth muscles, a longer upper lip, or if there is any skeletal outgrowth in the jaw.
This relaxes the upper lip muscle and prevents it from rising too high when you smile thereby keeping the gums covered.
When the gum exposure is more than a certain limit- maxillofacial surgical procedures are done to actually reset the maxilla bone backwards