I don't have relationship songs. Don't think I ever have. But there are some that definitely make me thing of somone.
Sophie Zelmani - I'll Remember You. It doesn't entirely apply, but I get in moods and then it almost does... and this song kills me.
Also, on a happier note: Blessid Union of Souls - I Wanna Be There. Best song. Applies entirely.
God, I'm fucking lame.