What are the reasons Martin Luther left the Catholic Church?

Martin Luther (November 10th, 1483, to February 18th, 1546) was a Roman Catholic priest and member of a monastic order who became a founder of the Protestant Reformation.
Luther's original aim was not to leave the Roman Catholic Church but to reform it from within. There were a few initial areas he considered urgently in need of reform. His first quarrel with the Church was over the sale of indulgences. Indulgences were based on the notion that saints had stored up an excess of merit beyond what was needed for their own salvation in a sort of metaphysical treasury of merits and that the Church could dispense these excess merits as "indulgences" to shorten other people's time in Purgatory. So, for example, an adulterer might be able to buy a new roof for the parish church, and in return (in theory) a bishop could ensure that he would not spend time in Purgatory for his sin. Luther believed that this system did not work and was immoral.
After a long dispute with the papacy, Luther was excommunicated and began to build his own theology involving salvation by faith and a more scriptural approach to religion.

It is important to remember that Martin Luther did not originally want to leave the Catholic Church, he wanted to reform it. As a result, he was forced out of the Church. Luther, a monk and former law student, was appalled at the corruption he saw in the Church. He saw Church leaders living immoral lives and using the religion to increase their wealth. What particularly pushed Luther to speak out was the selling of indulgences. These were payments that Catholics could make to the Church to absolve sin and limit the time they or someone else spent in purgatory. Luther saw this as a blasphemous moneymaking scheme that took advantage of faithful but naive Catholics.
In 1517, Luther publicly stated his complaints against the Church when he posted his "Ninety-Five Theses." In this, he denounced the sale of indulgences and explained the biblical source of salvation. He publicly urged Church leaders to abandon their corrupt practices. This was a dangerous move on Luther's part. Previous reformers had been declared heretics and executed.
The Church ordered Luther to recant, but he refused. At the Diet of Worms in 1521, Luther was excommunicated from the Catholic Church and declared a heretic. Forced out of the Catholic Church, Luther went on to start his own separate denomination known as the Lutheran Church. This new denomination sought to abandon the worldly trappings and the non-biblical tenets of the Catholic Church.

Martin Luther became dissatisfied with aspects of the Catholic Church after becoming a monk.  He saw corruption and immoral lifestyle choices among leaders in the Catholic Church.  He disapproved of this.  Martin Luther began studying the Bible more closely.  Through his studies, his beliefs experienced a shift.  Martin Luther came to believe that salvation came through faith alone, rather than by following the teachings of the Catholic Church.
In 1517, Martin Luther wrote his 95 Theses in protest of the Pope's approval of the sale of indulgences.  His 95 Theses mainly centered on indulgences.  The Pope needed to raise money, so indulgences were exchanged for alms.  Indulgences were purchased for forgiveness of sins.  It was unusual at this time for people to protest the Catholic Church, which was strong and dominant.  It was especially unusual for someone who had been a monk to protest.  Martin Luther's actions led to the beginning of the Protestant Reformation.