Disclaimer: The standard. I own a cheap sonic screwdriver from ThinkGeek (which is made of win) but all the characters are copyright to RTD, the BBC, et. al.
Why is she different? His Rose. Was this face more open to relationships? Was it merely that he was broken and she came at the right time to pick up the pieces, or try to. Was it her - her personality, her faith in him, or something in her silly human DNA that made them compatible. What a horrid word: compatible, like software in an operating system. Rose isn't programming, isn't lines of code. She's human, so human. She laughs, eats chips, likes the wrong bloke, makes mistakes, saves the world.
Why her? She's his plus one, no explanations necessary.