Its a good question, Solid, and I'm glad you asked.
The point here is that the celebration of birthdays is just a party to commemorate the day we were born. It was a festival invented by non-muslims, with pagan origins.
But Islam has given us better celebrations, and a higher aim than this. We celebrate the praises of our Lord on our Eids and during Ramadan. There are other days in the Islamic calendar that we can celebrate as well. So what need do the Muslims have to adopt the customs of those who have rejected their Creator?
Many people misinterpret this as saying that Islam is against having fun, etc. But that's not true at all. You can have parties and cakes, but we should do it in the Islamic tradition, and not over pointless occasions that have pagan origins.
The sad thing is Muslims are very good at throwing parties on birthdays and other non-muslim holidays, but they are much quieter in celebratoing Eid or Ramadan, when we should be having the most fun!
I hope I explained. Please ask if anything is not clear, or if you have any futher concerns.
