This is a question I often ponder about. I was asked this question some five years ago, by a philosopher. His intention was not to tease me. He just wanted to know why we celebrate our birthdays. He went on to say, "After all, a birthday is what each of us has." I, as a student philosopher, was impressed by this question and have often thought about it ever since.
Let me clarify his argument. It can be summarised this way.
(a) What is worth celebrating is something rare.
(b) We all have our birthdays.
(c) Therefore, birthdays are not rare.
(d) Therefore, birthdays are not worth celebrating.
So far as I can see, this argument is valid. That is, if the three premises, (a), (b), and (c), are true, then the conclusion (d) is inevitably true. Therefore, if we want to refute this argument, we have to show that some of the three is not true. I want to do that in other articles.