WHY THIS SCHOOL
Good answers: "Fit," you've spoken with students and really like them, you feel like you have a lot in common with them. You're interested in something specific that the school is known for (e.g., consulting at Tuck, finance at Stern, entrepreneurship at Babson, healthcare at Wharton). You enjoy the school's teaching model (case-method, group-work). You find the school's general attitude appealing (Wharton-competitive; Tuck-collaborative; Duke-tons of fun).
Bad answer: The weather where the school is located.
WHY DO YOU WANT AN MBA
Good answers: It makes sense given your career goals. You want to advance in your current track, and an MBA will help. You want to transition into another industry and an MBA will help. You want a broader skill-set and an understanding of various business disciplines.
Bad answers: You don't know what you want to do with your life. You think b-school will be like the Apprentice, your favorite show.