Birthdays are events that intensify one's focus on self. Birthdays are seldom used as occasions for offering 'thanksgiving' to the giver of all things. Instead, birthdays server as occasions for self indulgence and gratification. It is this fact that makes birthday celebrations to be evil.
Furthermore, we have not a single record in the Bible of a birthday celebration. Celebrating one's birth is a pagan custom, and we are to shun all forms of paganism.