While it is difficult to find exact reasons for the rise of the Ottoman empire, except that there must have been skilled leaders, sufficient economic backing, and probably weaknesses among the enemies; it is much easier to point at when the fall of the empire commenced, and its causes.
But let’s start with the rise: There are 2 main explanation theories. The first is that the community of the Turko-Mongol warriors had grown strong enough to exercise its force.
The good organization leads to a number of victories that could be further exploited to even more victories and gain of land.
The actual rise of the empire was gradual, and for half a century their own forces were enough to gain more land and then keep it. And as the indigenous forces proved too small, a system of forced conscription stated, but with effective recompense for the involved.
The second theory states that religious zeal made the Muslim warriors wage a jihad, holy war, against the Christian Byzantine empire in the west. While this theory has a lot of value for explaining the start of the empire, it cannot explain how the empire continued to grow over the next 4 centuries.
There are numerous reasons for the decline of the Ottoman Empire. The dominating reason, is that neighbour powers had grown stronger over the centuries. They had built stronger institutions, introduced modern arms, infrastructure and administration.
On the Ottoman side, however, many things had frozen in old structures, development on Ottoman territories was limited.
More than that, Ottoman institutions were often not working as smoothly as they used to.
But there were more things not working as they should have.
The sultans since Süleyman had often been less apt to their role; more and more power had moved into the hands of the kapikulli class; the trade routes running through the empire was no longer as important for Europe, and hence yielded less income; the population had grown big and had become less controllable; cities had become weaker. By the time of the 19th century, the European mock name of the empire was correct: The sick man of Europe.
The Ottoman Empire tried to correct all the weaknesses, but it proved to be too late. It was during the times of the reform process, Tanzimat, that the empire lost the most of its territory.