Because we have the chance to start fresh.
Traditionally, the Roman calendar began the first day of March. However, it was in January (the eleventh month) when the consuls of ancient Rome assumed the government. Julius Caesar, in 47 BC, changed the system, creating the Julian calendar, with some modifications in the time of Marco Antonio consul in 44 BC, the emperor Augustus Caesar in 8 BC and finally by Pope Gregory XIII in 1582, is used today. This year begins on January 1. Subsequently, the Gregorian calendar had the habit and the celebration was marked with a religious significance during the Middle Ages and later centuries.