There are many reasons that cause insomnia:
1) medical reasons (chronic pain, allergies, asthma, arthritis, neurological diseases such as Parkinsons),
2) psychiatric conditions (depression and anxiety for example),
3) lifestyle (working at home until late in the evening, taking short naps during the afternoon, if you work night shifts),
4) food. Certain types of food disturb your sleep (alcohol, coffee, cigarettes, heavy meals)
5) Some people are more prone to have insomnia. Without any particular reason, there are many possible chemical interactions in the brain of these people that could disturb sleep.