Knowing the Tin Man's origins may help you understand him. The Tin Man was once a human woodsman who fell in love with a Munchkin girl and wanted to marry her. However, the Wicked Witch of the East wanted to prevent the marriage, so she enchanted the woodsman's axe so that it chopped his leg off. Luckily, the woodsman knew a tinsmith who was able to make him a new leg out of tin, so the woodsman continued his trade. Unperturbed, the witch enchanted the axe again and again, until it cut off all his limbs and his head. The tinsmith helpfully replaced them all with tin substitutes. Yet the woodsman was still determined to marry the girl. Furious, the witch enchanted the axe one last time, and it cut his torso in half. The tinsmith replaced the woodsman's torso, but was unable to replace the heart. Now completely made of tin, but without a heart, the Tin Man became cold and indifferent to the girl and the marriage never occurred. He wants a heart so he can rekindle his love for the girl and marry her.
Despite being without a heart, the Tin Man shows great sensitivity to the plight of others. Along the yellow brick road, he notices a wildcat chasing something fiercely. Upon closer inspection, the Tin Man realizes that the wildcat is chasing a tiny mouse. To save the mouse's life, the Tin Man decapitates the wildcat with his axe. After the mouse thanks the Tin Man profusely, he says, "Don't speak of it, I beg of you. I have no heart, you know, so I am careful to help all those who may need a friend, even if it happens to be only a mouse."
The Tin Man is capable of great bravery, especially if his friends are being threatened or harassed. At one point in his journey to Oz with Dorothy, Toto, the Cowardly Lion and the Scarecrow, they must walk through a forest. The Scarecrow leads the way, but as soon as he tries to enter, one of the trees comes to life, picks him up and throws him headlong into the others. Shaken, the Scarecrow tries to enter again near a different tree, but this tree also picks him up and throws him. Then the Tin Man marches up to one of the trees. When it tries to grab him, he bravely chops off the opposing branches with his axe, stunning the tree long enough to let the rest of the travelers enter the forest.
Afraid of Rust
The Tin Man is deathly afraid of becoming rusty. When Dorothy and the others first encountered him, he had rusted so much that he could not move; he had been chopping wood and had gotten caught in a rainstorm before he could reach his oil can. This fear also makes him very nervous about anything involving moisture, like crossing a river or crying. It is helpful to note that tin does not rust and is corrosion resistant, but you can ignore this detail for the sake of understanding the Tin Man better.