Earlier quoted context omitted.
> it feels like the year is born in the spring Then shouldn't the start of the year be when the year is "born"? Or alternatively, the end of the year when the year "dies"? January 1 is neither.
January 1 is so close to December 25 (when Jesus is believed to have been born) so if we wanted to count years since that -- and call it AD for "Anno Domini" (the Year of Our Lord) -- we could declare the year to begin on January 1st. Which is exactly what we did.
25 of December is close to the winter solstice (around 22nd), and I suspect it was ascribed to that day for astrological reasons; there are a few other astrological clues around that episode, most prominently, the Bethlehem Star.