At one point in the novel, when rejected by Becky, Tom seriously considers running away and plans his future career, from running wild with Indians to the honoured life of a soldier. Finally, he settles on being a pirate.

Now his future lay plain before him, and glowing with unimaginable splendour. How his name would fill the world, and make people shudder! How gloriously he would go ploughing the dancing seas, in his long low black racer, the Spirit of the Storm, with his grisly flag flying at the fore! (chapter 8)

As is clear from this quote, the life of a pirate seems to Tom to be an especially daring, free, and glamorous one, affording the ultimate chance for adventure. He yearns for excitement, far away from the respectable stuffiness of his little hometown of St Petersburg.

Later in the story Tom coaxes his close friends Joe Harper and Huck Finn into pretending to be pirates also, as they spend a few glorious wild days on Jackson's Island, far removed from the constraints of civilisation. Jackson's island is one spot where the boys can give their imaginations full rein.

In the book The Adventures of Tom Sawyer, why does Tom decide to become a pirate?why does he become a pirate why does he run away

The answer to this question is at the beginning of Chaper 13. His "adventures" so far have just gotten him in trouble. His aunt is angry with him because he fed pain medication to the cat, and Becky, when she returns to school, speaks scornfully to him after he tries to get her attention by showing off.  He sees himself as "gloomy and desperate" (98). He believe nobody loves him, and so he sets out "to lead a life of crime (98). When he meets his friend Joe, Joe's idea to escape their troubles is for them to become hermits.  But Tom persuades Joe that there are "conspicuous advantages" to becoming pirates, and that is what they decide to do. Then they went to find Huckleberry Finn, who was happy to become a pirate, too. They had no idea what kind of crimes they would commit as pirates, but they decided to start out at midnight on the river bank.