Why did Jefferson Davis find it necessary to attack Fort Sumter?

He found it necessary because Lincoln was only transferring food to the Northern army at the fort. However, once the ship came to the fort, it was believed (or assumed) by the south that inside the ship were supplies of weapons, soldiers, anything besides food. The South then opened fire, catching the North off guard and the first battle of the Civil war began.

The fort protected the New Orleans harbor.