Alex and Izzie

Because he didn't know her back then. But now he does.
--> Because he's the bad boy and she the sweet model.
--> Because he is jealous when Izzie is with another guy.
--> Because he still had feelings for her
--> Because she didn't want Alex to mover in considering their past.
--> Because they're hot.
--> Because they amor each other.
--> Because they are friends.
--> Because they both let Cristina of the hook.
--> Because she orders everything he said.
--> Because Izzie is the first one in the hospital that sees that Alex is okay.
--> Because they blew away an eyelash...