Ingrown toenails in reality have only two causes.  People are either born with an ingrown nail or it develops later in life due to gradual damage to the cells that grow the nail.  Nails grow from an area of tissue called the nail matrix, located at the base of the nail just under the skin.  Also known as the nail root, this tissue forms and pushes nails outward.  Nails can grow curved inward toward the skin from this area sometimes, either due to an abnormality of the cells from birth or eventual damaging pressure from tight shoes or toe injuries. Repetitive injuries to the toe, such as heavy objects falling it, pressure from poorly fitting shoes across the top of the toe, nail fungus creating thickness and loosening, or toe bruising (common in athletes) may cause irreversible changes to the nail matrix by causing undue pressure to these fragile cells.  In time, the nail may abnormally grow inward toward the skin due to these changes, rather than flat outward as in normally shaped nails. It is possible that a poor nail trimming technique may leave a spike of nail that may protrude into the skin as the nail grows outward, although this is very uncommon.

More times than not,  people get ingrown nail symptoms after cutting the nail too short.  Usually people nick the skin in some way, starting a skin inflammation process that eventually causes the pain by allowing the sensitive swollen skin to push against the ingrown nail side.  This is really the underlying problem with ingrown nail pain.  The nail itself is not the source of the pain as the skin is generally well suited to accept a curved nail next to it.  The pain arises when inflammation to the skin along side the nail develops.  The inflammation is painful, not the nail itself.  Unfortunately, in many people, this inflammation begins to cycle regularly, with little to set it off, including tight shoes, an injury, a slightly close nail trim, et cetera.  Eventually an infection can and will develop over time as bacteria becomes trapped under the swollen skin.  If untreated, the chronic infection may spread along the toe skin and in very extreme cases may eventually involve the toe bone underneath, although this is very uncommon.  Ingrown nail symptoms may resolve on their own, only to keep reappearing over and over again.