I do think the time of year that someone is born, and the resultant times of the year they pass through key early milestones, can have a lasting and slightly predictable effect on their personality lifelong. I'm a very typical Capricorn by most accounts, and I know that the fact that I've always celebrated and looked forward to my birthday in the coldest, darkest time of year has had a profound effect on how I see my place in the world.
The Zodiac is simply a proto-calendar: a loop of star patterns in the sky that changes predictably depending on the time of year. It's like saying the position of the hands on a clock have a connection to my state of alertness. It's not the hands themselves that directly tweak my mood, it's what they stand for (the time of day).
I do think there is something to be said, also, for the symbolism of astrology being a helpful tool for self reflection. Would I base key life decisions on what my horoscope said? Probably not. But can it be interesting food for thought, and a good excuse for a bit of self-reflection as I thumb through the morning paper? Sure.