From the Self news article:
Ingrown toenails happen when the toe grows down instead of out, leading it to dig into the surrounding skin. Hence why they are normally very uncomfortable. This most commonly happens in the big toes, and can be caused by anything that leaves the nail deformed. “Common causes include ill-fitting shoes where the toes are cramped within the shoe, trauma to the nail (i.e., a fallen object or collision/impact) causing nail deformity, and nail fungus resulting in thickened nails,” Quinton Yeldell, D.P.M., founder of foot care brand Southern Hospitality, tells SELF. (Aren’t feet just the sexiest?)
Read more at the link:
http://www.self.com/story/ingrown-toenails-treatment-prevention