There is a widespread misconception (based on Sanhedrin 63) that the only reason why people leave observance is to indulge in their desires. In general, we can say that there are plenty emotional and rational reasons why people leave Orthodox Judaism.
Rational reasons include:
Lack of belief in the notion of God
Lack of belief in the authority of Torah sages (‘Daas Torah, ‘Emunat Chachamim’)
Disagreement over halachic issues, such as the status of women and non-Jews in halacha
Conflicts with science (age of the world, e.g.)
Questioning dogma and beliefs in general
Some emotional reasons could include:
Never having felt a spiritual connection to a God
Being fed up of always having to be obedient and subservient
A general or specific desire for freedom (non-kosher food, away from community pressure, sexuality)
Abuse (emotional or sexual)
A general feeling of unhappiness or burnout connected or not connected to religion
Exposure to more tolerant forms of experiencing spirituality
To learn more about some reasons why people leave observant Judaism, the book Off the Derech by Faranak Margolese can be helpful, although many ex-observant people disagree with some of the premises and conclusions in the book. Nevertheless, it is the only book so far that seems to address the issue of why people leave orthodoxy.
http://www.offthederech.org/wp-content/uploads/2017/07/otd.png00Chatzkaleh Koferhttp://www.offthederech.org/wp-content/uploads/2017/07/otd.pngChatzkaleh Kofer2017-05-26 08:28:542017-05-26 08:28:54Reasons Why People Go OTD