Sheldon goes home to Texas to help with his sister's birth. He doesn't handle the home birth well at all.  In his absence, his friends have a Christmas party and trim the tree without having to worry about Sheldon's rules. They consider how their lives would have been different if Sheldon wasn't in their lives. They guys would have still worked together but none of them would have their current girlfriends. And Amy would never have been set up with Sheldon. Sheldon brought them all together.