He’s never picked up a needle in his entire life, so threading it seems like a legitimate impossibility. He might as well be trying to run a rag through the tiny hole, but even he knows if there’s a will; a conviction that he promises and keeps to himself, that there’s a way.
So once he starts pulling, he doesn’t stop, even when it gets stuck fast in the edges. There’s no other way he can accept, except to commit to the challenge and tell himself there’s no way he’s going back. And when he finally pulls through, it pulls through for him, and he realises that the rag had to unravel before it would thread through the eye of the needle.