1. You're uncomfortable.
2. You've never been properly fitted.
3. You buy the cheapest bra in the store.
4. Or you buy the prettiest bra in the store.
5. You machine wash your bras.
6. You've been wearing the same bra for forever.
7. Your body shape has changed but not your bras.
8. You're unsatisfied with your breasts' size and/or appearance.
Of course, wearing the so-called "right" bra size isn't mandatory. If you can wear a "wrong" size comfortably, then more power to you. If you want to go braless, go for it. Whatever the case, you do you.