I go along with a minimal "celebration" of mine for one reason: Because I want to celebrate other people's, particularly my children's. Sometimes, too, once people's mothers are gone their relatives celebrate their birthdays since their mother isn't around to put together a celebration.
The days that each of my three children came into this world were days that changed my life and added so much to it. While I had two of my kids myself, I wasn't even aware that my eldest son was being born on the day he came into the world. Still, his coming into the world would bring so much to my life, change it, contribute to my own growing over the years - just as with the day that my two younger kids were born. So, particularly for mothers/fathers, the day is one to celebrate.
Also, though, and aside from that; it's just kind of a nice thing to do, to give someone "their special day" and have their family members and/or friends do something nice for them.
A whole lot of adults that I know, including myself, don't care about their own birthdays; but if you're going to be a "troll" about your own birthday you can't expect those whose birthdays you do sincerely want to celebrate to overcome their own "non-self-centeredness" and go along with the celebration you want to have.
Besides, it's a tradition and it's just kind of nice. If we didn't celebrate things here and there, every day and week and month and year would all be the same. Doing something nice just because we're people, and even the most reserved of us are usually fairly "social" creatures; traditions and little social events can be a more important part of life than one might think.
And on a more somber note, the more of life we see, the more we realize that we can't take for granted each birthday that passes. And so, when all is said and done, maybe we celebrate birthdays because being alive is, in fact, something to celebrate.