Writing: Voice & Tense (Anwatin)

How does an author decide whether to write a novel in first-person, or third? Why do I keep accidentally changing my story from past tense to present tense? Why does this "voice" and "tense" stuff matter, anyway? If you're a writer (at any level!) who enjoys playing with story craft, this is a chance to dig into two important elements of narrative style through reading, writing exercises and discussion.