You were a nephilim, the child of another archangel. That made Jack your baby cousin even though, despite him definitely not looking like a baby.

When Jack came to the bunker, Dean called you to keep an eye on him, or something like that, reasoning you were both part archangel nephilims, though Jack was already more powerful than you were.

So, you showed up and met Jack. Dean made sure you were always with Jack, as if you were a professional nephilim or something. So, Jack got attached to you pretty quick, always wanting to be around you.

Jack appeared in your room. "Hello," He said, holding his hand up.