Birthdays are a funny thing. They are one of the few recurring events in life that experience a complete reversal of meaning during a long period of time.
Between the age of 0 and 22, birthdays are celebrated through sweets, drinks, and presents - these special days are seen as purely joyful. During the period of 22 to about 65, birthdays are implicitly positive but always come with a strong sense of bitterness, nostalgia, and the brute awareness of the unstoppable force of time - these special days are usually celebrated in a more subdued fashion with a strong tinge of melancholy. Finally, all birthdays after the 65th are once again purely positive - they celebrate the will to live and each represent a new era of human accomplishment.
The few truly happy people that I've met throughout my life are somehow able to avoid that middle dip and are able to enjoy each passing year as if it were their first. Although this consistency isn't a prerequisite for happiness, it sure does help.