Happy birthday! may this be a year of growth and realizing your potential.
As far as why, I can think of a few reasons. For starters, when we talk about certain days having "shefa," abundance or special energy flow from hashem, well, today is certainly the of abundance, of energy for you - it's the day that everything about yo came into being!
Also, since it's the day of beginning for you, it's an opportunity to focus on your priorities in the coming year. And anytime we have a big opportunity, the consequences of (and response to) our choices is equally big.