1. You were given birth to in sin: (For I was born a sinner yes, from the moment my mother conceived me. Psalms 51:5) Why celebrate a day you were born into sin and a day you were born into a wicked world?

2. It stems from pagan origin: (So do not act like the people in Egypt, where you used to live, or like the people of Canaan, where I am taking you. You must not imitate their way of life. Leviticus 18:3) 

The ancient Egyptians and Babylonians are the ones that celebrate birthdays. The Egyptians kings actually deify themselves. So they celebrate with a great feast into godhood on their birthdays. Romans borrowed it from pagan Egypt and Babylon, before it's now westernized. Self-worship is abomination to God.

3. The people of the book (Israelites) DO NOT celebrate birthdays: where have you seen in the bible? Where the Israelites celebrate birthday? Or Gods prophets and servants? Jacob? Samuel? David? Paul? Peter or even Jesus? Only Herod, Herod was not really a Jew, the Herodians have Pagan Roman influence. (Matthew 14:6-13) God did not prioritize celebrating birthdays.  

What you should do on your birthday:

(So teach us to number our days that we may get a heart of wisdom. Psalm 90:12). Your birthday should be a day of sober reflection, meditation and a day to commune with God.

A day you should celebrate:

(In the same way, there is joy in the presence of God's angels when even one sinner repents. Luke 15:10) A day a believer should celebrate is the day he/she accepts the savior.