Pauline was Mario's Girlfriend in the early Mario games. Princess Toadstool was not Mario's girlfriend, just the princess of The Mushroom Kingdom. Pauline, I guess, broke up with Mario because she probably thought Mario liked Princess Toadstool (Remember that I'm guessing). What I also have to ask, as a message board question, is Why did Princess Toadstool change her name to 'Peach?'