Machiavelli wanted to be a Prince. But nobody would hire him.
Actually, Machiavelli was a dyed in the wool republican who hated the Medici and was incredibly disappointed when Florence threw away it's republic and welcomed the Medici back into power
Some authors have postulated that the dark and cynical tone of "The Prince" reflects this disappointment and a resultant general cynacism with human nature.
It's not that complicated, though. Although Niccolo felt that a republic (of the old Roman form) was the best form of government by far (this is clear in his real political masterwork "The Discourses on the first ten books of Titus Livius"), Machiavelli loved the idea of govenment and the workings of government more than any particular form of government.
In that sense, he was a lot like some guys you see in Washington today (David Gergen comes to mind, though probably not the best example), who land advisory positions in administration after administration, without regard to the party in power.
So, when the Medici came back into power and he was unemployed (he had had a number of roles in the Florentine Republic), he did what anybody does when trying to get a job - he wrote a resume tailored to the target employer. Since he was trying to get hired by a prince, and was trying to demonstrate everything he could do for him, he wrote - "The Prince".