What inspired Thomas Paine to write Common Sense?

Thomas Paine’s Common Sense was first published in January 1776, amid the American Revolution. It was inspired by the American colonies’ rebellion against taxation—which turned into a struggle for outright independence—from Britain. Given that progressive social and political change (in the form of constitutional and republican governments, as opposed to monarchical systems) was a general topic of debate among elite western European and American politicians and political commentators, it is likely that Paine was also inspired by this overarching political debate over the very nature of government and revolution. In other words, Paine was likely also inspired by Enlightenment ideas and ideals at large. It is also possible that Paine was inspired by his own background. Rather than being born into power, influence, and money, he was of humble origins and had to struggle to educate himself and gain employment. In fact, this is partially why he immigrated to America, getting a job as an editorial assistant and writer for the Pennsylvanian Magazine in Philadelphia. It was there that he became acquainted with and involved in American politics and was inspired (and encouraged by colleagues) to write Common Sense.
In this pamphlet, Paine advocated for American independence from Britain and for a republican form of government. In the introduction, he wrote, “As a long and violent abuse of power, is generally the Means of calling the right of it in question . . . and as the King of England hath undertaken in his own right, to support the parliament in what he calls theirs, and as the good people of this country are grievously oppressed by the combination, they have an undoubted privilege to inquire into the pretensions of both, and equally to reject the usurpation of either.”
The pamphlet was immensely popular and written in a way that was accessible to all readers, not simply elite political thinkers. Therefore, it really was inspired by the American Revolution (and, in turn, inspired the revolution itself). Some have even referred to Paine as the world’s first international revolutionary.
For more information, I would recommend reading Common Sense itself, as well as Mark Philp’s introduction to the Oxford World’s Classics Thomas Paine: Rights of Man, Common Sense, and Other Political Writings.

During the American Revolutionary War, many colonists were eager to separate from England. Some people were still considering a rapprochement with England, however, and the leaders of the revolution were searching for a way to encourage the colonists that reconciliation would be a mistake.
In early 1776, Thomas Paine penned Common Sense, using plain language and persuasive prose to convince the colonists that independence was the only viable choice. After the pamphlet was circulated, popular opinion shifted, and even George Washington felt that Paine had been the deciding factor.
Before reading the pamphlet, many people still thought of themselves as British citizens and hoped for a chance to rejoin their homeland. After Common Sense, the people hungered for change and demanded a nation free from unfair taxation and the oppression of the monarchy.
https://www.thomaspainesociety.org/common-sense