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In Mark Twain's The Adventures of Huckleberry Finn, I would assume that Huck Finn is a hard worker because he must endure—stay alive.
It is true that Huck has been living with the Widow Douglas and Miss Watson, where his physical needs (food, clothing, shelter) were seen to. However, Huck, although only fourteen, knows he cannot be happy with these women as they try to civilize him. He is bright enough to learn to read and write, which will aid him in the world at large, but he cannot try to be what others want for him, only what he sees he must have. Huck's biggest need is to be free.
Once Huck and Jim, Miss Watson's escaped slave, take up life on the Mississippi River, their very survival depends upon working hard. They must make sure to keep the raft afloat, avoid others who might try to take the raft, as well as stay safe while on land. They need to eat. Huck and Jim must be careful and vigilant: they meet some unsavory characters, who are up to no good. And Jim is a runaway slave, which could draw unwanted attention, especially if someone wanted to steal Jim away for a reward, etc.
Though Huck may seem simply a free spirit, he is a young man of conviction, knowing by his own internal code what is morally right, and what he must do to protect his friend Jim. He needs to guarantee that both of them come through their adventures and ordeals safely. Freedom comes at a price, and it is only through hard work that Huck—and Jim—survive.