The sweater cannot speak, it's an object, but for some reason, it has an effect on people, it'll make them want to wear it. When they do, it'll seem soft and comfortable. But over time, it grows, threads out of nowhere will remove and replace other clothing on the person's body. It will do this until all their clothing is gone but it. Then it will feed threads into their mind, and make them into a slave to sweaters. At which point, they're doomed.