Wondering

sometime im really wondering why is there a birthday ? when the day of your birthday why is there a celebration ? when the day of your birthday if anything goes wrong you will feel x2 sad/boring/sick ? when the day of your birthday you wanted to get lots of presents ( and the whole year you will start giving birthday present for the vaule they give you ) ? why ? some people can just survive birthday with a bowl of egg noodle, some people don't have time to even remember their birthday, some people are lonely and birthday make them more lonely, some people can spend few hundreds of dollars just to celebrate his/her birthday ( problem is they haven even start working ) ... so what is the main purpose of celebrating birthday ?