Why did people celebrate birthdays?
First, you had been born. That was truly an incredible achievement. Time travel had prevented many children who should have been born from ever being born. More importantly, it was not your parents who had given birth to you, but the dead. After surviving anti-intellectual campaigns such as wars, plagues, natural disasters, famines, and witch hunts, they had still been capable of having children. That alone was already remarkable.
You were the lucky one among twenty-five million sperm cells, defeating 24,999,999 competitors. Add to that all the animals and plants humans had eaten, and there was no question that being born human rather than an animal like a pig or dog was an outstanding achievement.
Second, you were still alive. You had been fortunate enough not to die in a car crash, fire, or from falling objects. The 1310 rebellion had been stopped by a woman with a flowerpot. She had smashed the standard-bearer to death. Not only had she not been punished for dropping something from above, she had been praised for it. While American soldiers were eating sand in the Middle East, you could still sit down and eat cake. That was already very fortunate.
Third, you had plenty of food to choose from. That was the greatest happiness of living in peaceful times. During war, supplies were scarce. During the Little Ice Age, plants had very little time to grow. When volcanoes erupted, ash blocked out the sunlight, and fruit was out of the question, let alone using cake to start a food fight.
Those three things alone were worth celebrating. Usually, the people who carefully prepared for your birthday were your parents, especially your mother. In Petunia's eyes, the bully Dudley would always be an angel, while Harry had not felt