What is the purpose of celebrating a birthday?
Perhaps the answer is found in the verse that speaks about the one-year-old sheep that were brought as sacrifices to the Temple.
“One yearling sheep for a burnt offering” (Numbers 7:75).
In order to qualify as sacrifices, the priests would have to count the 365 days that elapsed from their birth to determine their exact age.
The priests would also evaluate their health, their weight etc.
This is the purpose of a birthday: to look back and count each and every day of the past year, to evaluate where one stands in life.
–Rabbi Adin Steinsaltz, on the occasion of his 75th birthday